Semiconductor device

ABSTRACT

A highly reliable semiconductor device including an oxide semiconductor is provided by preventing a change in its electrical characteristics. A semiconductor device which includes a first oxide semiconductor layer which is in contact with a source electrode layer and a drain electrode layer and a second oxide semiconductor layer which serves as a main current path (channel) of a transistor is provided. The first oxide semiconductor layer serves as a buffer layer for preventing a constituent element of the source and drain electrode layers from diffusing into the channel. By providing the first oxide semiconductor layer, it is possible to prevent diffusion of the constituent element into an interface between the first oxide semiconductor layer and the second oxide semiconductor layer and into the second oxide semiconductor layer.

TECHNICAL FIELD

The invention disclosed in this specification and the like relates to asemiconductor device and a method for manufacturing the semiconductordevice.

In this specification and the like, a semiconductor device refers to alltypes of devices which can function by utilizing semiconductorcharacteristics; an electro-optical device, an image display device, asemiconductor circuit, and an electronic appliance are all semiconductordevices.

BACKGROUND ART

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has beenattracting attention. Such a transistor is applied to a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (also simply referred to as a display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention.

For example, a technique by which a transistor is formed using zincoxide or an In—Ga—Zn-based oxide semiconductor as an oxide semiconductoris disclosed (see Patent Documents 1 and 2).

Non-Patent Document discloses a transistor having a structure in whichoxide semiconductors having different compositions are stacked.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

Non-Patent Document

-   [Non-Patent Document 1] Masashi Ono et al., “Novel High Performance    IGZO-TFT with High Mobility over 40 cm²/Vs and High Photostability    Incorporated Oxygen Diffusion”, IDW'11 Late-News Paper, pp.    1689-1690.

DISCLOSURE OF INVENTION

In a transistor including an oxide semiconductor, if a trap level (alsoreferred to as an interface state) exists at an interface between anoxide semiconductor layer and a layer in contact with the oxidesemiconductor layer, the existence of the trap level causes a change inthe electrical characteristics (e.g., the threshold voltage or thesubthreshold swing (s value)) of the transistor.

For example, in a bottom-gate transistor, when a constituent element ofa source electrode layer and a drain electrode layer diffuses into aback channel of an oxide semiconductor layer, the constituent elementforms a trap level, so that the electrical characteristics of thetransistor is changed. Further, the existence of a trap level at aninterface between the oxide semiconductor layer and a gate insulatinglayer might cause a change in the electrical characteristics of thetransistor.

Thus, an object of one embodiment of the present invention is to providea highly reliable semiconductor device including an oxide semiconductorby preventing a change in its electrical characteristics.

One embodiment of the present invention is a bottom gate transistorincluding an oxide semiconductor, which has at least a stacked structureformed of a first oxide semiconductor layer which is in contact with asource electrode layer and a drain electrode layer and a second oxidesemiconductor layer which is provided between the first oxidesemiconductor layer and a gate insulating layer. In the abovetransistor, the second oxide semiconductor layer is used as a maincurrent path (channel) of the transistor and the first oxidesemiconductor layer is used as a buffer layer for preventing diffusionof a constituent element of the source and drain electrode layers,whereby a change in the electrical characteristics of the transistor canbe prevented. Specifically, any of the following structures can beemployed for example.

One embodiment of the present invention is a semiconductor deviceincluding a gate electrode layer, a gate insulating layer over the gateelectrode layer, an oxide semiconductor stack overlapping with the gateelectrode layer with the gate insulating layer positioned therebetween,and a source electrode layer and a drain electrode layer electricallyconnected to the oxide semiconductor stack. The oxide semiconductorstack includes a first oxide semiconductor layer in contact with thesource electrode layer and the drain electrode layer and a second oxidesemiconductor layer provided between the first oxide semiconductor layerand the gate insulating layer. The first oxide semiconductor layercontains at least indium and gallium, and the content of the indium islower than or equal to the content of the gallium in the first oxidesemiconductor layer. The second oxide semiconductor layer contains atleast indium and gallium, and the content of the indium is higher thanthe content of the gallium in the second oxide semiconductor layer. Thefirst oxide semiconductor layer contains a constituent element of thesource electrode layer and the drain electrode layer, as an impurity.

Another embodiment of the present invention is a semiconductor deviceincluding a gate electrode layer, a gate insulating layer over the gateelectrode layer, an oxide semiconductor stack overlapping with the gateelectrode layer with the gate insulating layer positioned therebetween,and a source electrode layer and a drain electrode layer electricallyconnected to the oxide semiconductor stack. The oxide semiconductorstack includes a first oxide semiconductor layer in contact with thesource electrode layer and the drain electrode layer, a third oxidesemiconductor layer in contact with the gate insulating layer, and asecond oxide semiconductor layer provided between the first oxidesemiconductor layer and the third oxide semiconductor layer. The firstoxide semiconductor layer and the third oxide semiconductor layercontain at least indium and gallium, and the content of the indium islower than or equal to the content of the gallium in the first oxidesemiconductor layer and the third oxide semiconductor layer. The secondoxide semiconductor layer contains at least indium and gallium, and thecontent of the indium is higher than the content of the gallium in thesecond oxide semiconductor layer. The first oxide semiconductor layercontains a constituent element of the source electrode and the drainelectrode layer, as an impurity.

Another embodiment of the present invention is a semiconductor deviceincluding a gate electrode layer, a gate insulating layer over the gateelectrode layer, an oxide semiconductor stack overlapping with the gateelectrode layer with the gate insulating layer positioned therebetween,and a source electrode layer and a drain electrode layer electricallyconnected to the oxide semiconductor stack. The oxide semiconductorstack includes a first oxide semiconductor layer in contact with thesource electrode layer and the drain electrode layer, a third oxidesemiconductor layer in contact with the gate insulating layer, and asecond oxide semiconductor layer provided between the first oxidesemiconductor layer and the third oxide semiconductor layer. The firstoxide semiconductor layer and the third oxide semiconductor layercontain at least indium and gallium, the content of the indium is lowerthan or equal to the content of the gallium in the first oxidesemiconductor layer and the third oxide semiconductor layer. The secondoxide semiconductor layer contains at least indium and gallium, and thecontent of the indium is higher than the content of the gallium in thesecond oxide semiconductor layer. The first oxide semiconductor layercontains a constituent element of the source electrode and the drainelectrode layer, as an impurity. The third oxide semiconductor layercontains a constituent element of the gate insulating layer, as animpurity.

In any one of the above semiconductor devices, the source electrodelayer and the drain electrode layer preferably contain copper.

In any one of the above semiconductor devices, the gate insulating layermay include a silicon nitride film.

Effects of a structure according to one embodiment of the presentinvention can be described as follow. Note also that the followingdescription is just a consideration.

The transistor of one embodiment of the present invention includes thefirst oxide semiconductor layer which is in contact with the sourceelectrode layer and the drain electrode layer and the second oxidesemiconductor layer which serves as a main current path (channel) of thetransistor. Here, the first oxide semiconductor layer serves as a bufferlayer for preventing a constituent element of the source and drainelectrode layers from diffusing into the channel. By providing the firstoxide semiconductor layer, it is possible to prevent diffusion of theconstituent element into an interface between the first oxidesemiconductor layer and the second oxide semiconductor layer and intothe second oxide semiconductor layer.

Further, it is preferable that a metal oxide which is applied to thefirst oxide semiconductor layer have a larger energy gap (band gap) thana metal oxide which is applied to the second oxide semiconductor layerbecause a conduction band offset can be formed between the second oxidesemiconductor layer and the first oxide semiconductor layer. When theconduction band offset exists in the oxide semiconductor stack, carriersflow through the second oxide semiconductor layer without passingthrough the first oxide semiconductor layer and the interface thereof;thus even if a trap level due to the diffusion of the metal elementexists on the back channel side, the oxide semiconductor stack isinsusceptible to the trap level. Accordingly, the electricalcharacteristics of the transistor can be stabilized.

It is preferable that the transistor of one embodiment of the presentinvention further include a third oxide semiconductor layer which isprovided between the second oxide semiconductor layer and the gateinsulating layer and is in contact with the gate insulating layer, inaddition to the first oxide semiconductor layer and the second oxidesemiconductor layer. The third oxide semiconductor layer contains one ora plurality of metal elements selected from the constituent elements ofthe second oxide semiconductor layer, and the third oxide semiconductorlayer includes characteristics of the second oxide semiconductor layer.Therefore, when the third oxide semiconductor layer is provided, theinterface on the gate insulating layer side of the second oxidesemiconductor layer functioning as the channel can be stabilized. Thatis, the third oxide semiconductor layer functions as a buffer layer forpreventing degradation of the interface. In particular, photodegradation(e.g., negative-bias temperature stress photodegradation) of thetransistor can be reduced by prevention of capture of carriers at theinterface on the gate insulating layer side of the channel, so that thetransistor can have high reliability.

Further, it is preferable that, in a manner similar to the metal oxideapplied to the first oxide semiconductor layer, a metal oxide which isapplied to the third oxide semiconductor layer have a larger energy gapthan the metal oxide applied to the second oxide semiconductor layerbecause a conduction band offset can be formed between the third oxidesemiconductor layer and the second oxide semiconductor layer. Also in ageneral MISFET, a trap level or the like is generated at an interfacebetween a gate insulating layer and a semiconductor and the electricalcharacteristics of the FET is degraded; however, when a third oxidesemiconductor layer is provided, the MISFET has a structure in whichcarriers flow through a region which is apart from the gate insulatinglayer (what is called a buried channel), and therefore influence of theinterface can be reduced.

In the case where metal oxides which are formed of the same constituentelements but have different compositions are applied to the first oxidesemiconductor, the second oxide semiconductor, and the third oxidesemiconductor, for example, metal oxides containing at least indium andgallium can be used as the first oxide semiconductor, the second oxidesemiconductor, and the third oxide semiconductor. Here, as the ratio ofthe indium content to the content of the other metal element(s) becomeshigher in the metal oxide, the field-effect mobility of the metal oxideis increased; as the ratio of the gallium content to the content of theother metal element(s) becomes higher in the metal oxide, the energy gapof the metal oxide is increased. Therefore, a metal oxide in which theindium content is higher than the gallium content is preferably used asthe second oxide semiconductor to be a channel formation region, andmetal oxides in which the indium content is lower than or equal to thegallium content are preferably used as the first oxide semiconductor andthe third oxide semiconductor which function as buffer layers.

According to one embodiment of the present invention, a change in theelectrical characteristics of a transistor including an oxidesemiconductor can be prevented, so that a highly reliable semiconductordevice can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are a plan view and cross-sectional views illustratingone embodiment of a semiconductor device.

FIGS. 2A to 2D are a plan view, cross-sectional views, and a banddiagram illustrating one embodiment of a semiconductor device.

FIGS. 3A to 3D illustrate an example of a method for manufacturing asemiconductor device.

FIGS. 4A to 4C each illustrate one embodiment of a semiconductor device.

FIG. 5 illustrates one embodiment of a semiconductor device.

FIGS. 6A and 6B illustrate one embodiment of a semiconductor device.

FIGS. 7A and 7B illustrate one embodiment of a semiconductor device.

FIGS. 8A to 8C illustrate electronic appliances.

FIGS. 9A to 9C illustrate an electronic appliance.

FIGS. 10A and 10B are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 11A to 11C illustrate one embodiment of a semiconductor device.

FIG. 12 shows measurement results of SSDP-SIMS in an example.

FIG. 13 illustrates one embodiment of a semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and an example of the present invention will bedescribed in detail with reference to the accompanying drawings. Notethat the present invention is not limited to the description below andit is easily understood by those skilled in the art that the mode anddetails can be changed in various ways. Therefore, the invention shouldnot be construed as being limited to the description in the followingembodiments and example.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and description thereofis not repeated. Further, the same hatching pattern is applied toportions having similar functions, and the portions are not especiallydenoted by reference numerals in some cases.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that the ordinal numbers such as “first” and “second” in thisspecification and the like are used for convenience and do not denotethe order of steps or the stacking order of layers. In addition, theordinal numbers in this specification do not denote particular nameswhich specify the present invention.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and oneembodiment of a method for manufacturing the semiconductor device aredescribed using FIGS. 1A to 1C, FIGS. 2A to 2D, and FIGS. 3A to 3D. Inthis embodiment, a bottom-gate transistor including an oxidesemiconductor layer is described as an example of the semiconductordevice.

FIGS. 1A to 1C illustrate a structure example of a transistor 300. FIG.1A is a plan view of the transistor 300, FIG. 1B is a cross-sectionalview taken along dotted line X1-Y1 in FIG. 1A, and FIG. 1C is across-sectional view taken along dotted line V1-W1 in FIG. 1A.

The transistor 300 includes a gate electrode layer 402 which is providedover a substrate 400 having an insulating surface, a gate insulatinglayer 404 over the gate electrode layer 402, an oxide semiconductorstack 408 which is on and in contact with the gate insulating layer 404and overlaps with the gate electrode layer 402, and a source electrodelayer 410 a and a drain electrode layer 410 b which are electricallyconnected to the oxide semiconductor stack 408. Note that an insulatinglayer 412 which covers the source electrode layer 410 a and the drainelectrode layer 410 b and is in contact with the oxide semiconductorstack 408 may be included in the transistor 300 as a component.

The oxide semiconductor stack 408 in the transistor 300 includes anoxide semiconductor layer 408 a which is in contact with the sourceelectrode layer 410 a and the drain electrode layer 410 b, and an oxidesemiconductor layer 408 b which is provided between the oxidesemiconductor layer 408 b and the gate insulating layer 404.

The oxide semiconductor layer 408 b in the oxide semiconductor stack 408is a region serving as a channel of the transistor 300. The oxidesemiconductor layer 408 a provided on the back channel side of the oxidesemiconductor layer 408 b functions as a buffer layer for preventing aconstituent element of the source electrode layer 410 a and the drainelectrode layer 410 b from diffusing into the oxide semiconductor layer408 b. That is, the oxide semiconductor layer 408 a contains theconstituent element of the source electrode layer 410 a and the drainelectrode layer 410 b, as an impurity. By providing the buffer layer,formation of a trap level in the channel of the transistor 300 can beprevented; thus an increase in s-value due to the trap level can beprevented. Accordingly, variations in the electrical characteristics ordegradation over time of the transistor can be prevented; thus a highlyreliable semiconductor device can be provided.

For the oxide semiconductor layer 408 a and the oxide semiconductorlayer 408 b, oxide semiconductors which have different constituentelements or oxide semiconductors which have the same constituentelements but whose compositions are different from each other may beused. Note that it is preferable that an oxide semiconductor which hashigh field-effect mobility be used for the oxide semiconductor layer 408b functioning as the channel of the transistor 300.

For example, in the case where the oxide semiconductor layer 408 a andthe oxide semiconductor layer 408 b are formed to have the sameconstituent elements by using oxide semiconductors which contain atleast indium and gallium, an oxide semiconductor in which the indiumcontent is larger than the gallium content is preferably used for theoxide semiconductor layer 408 b, and an oxide semiconductor in which theindium content is lower than or equal to the gallium content ispreferably used for the oxide semiconductor layer 408 a.

In an oxide semiconductor, the s orbital of heavy metal mainlycontribute to carrier transfer, and when the percentage of indium in theoxide semiconductor is increased, overlaps of the s orbitals are likelyto be increased. Accordingly, when the indium content is higher than thegallium content in the oxide semiconductor layer 408 b, it is possiblethat the oxide semiconductor layer 408 b has higher field-effectmobility than an oxide in which the indium content is lower than thegallium content.

Further, as the ratio of the gallium content to the content of the othermetal element(s) becomes higher in a metal oxide, the energy gap of themetal oxide is increased. Accordingly, when the indium content is lowerthan or equal to the gallium content in the oxide semiconductor layer408 a, the oxide semiconductor layer 408 a has a larger energy gap thanthe oxide semiconductor layer 408 b. Accordingly, the above structure ispreferable because a conduction band offset can be formed between theoxide semiconductor layer 408 b and the oxide semiconductor layer 408 a.Further, gallium needs large formation energy of an oxygen vacancy andthus is not likely to generate an oxygen vacancy as compared to indium.Therefore, a metal oxide in which the indium content is lower than orequal to the gallium content has stable characteristics as compared to ametal oxide in which the indium content is higher than the galliumcontent. Therefore, stabilization on the back channel side in thetransistor 300 can be achieved.

For example, in the case where an In—Ga—Zn-based oxide semiconductor isused for the oxide semiconductor layer 408 a and the oxide semiconductorlayer 408 b, an In—Ga—Zn-based oxide having any of atomic ratios ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=1:3:2 (=1/6:3/6:2/6),In:Ga:Zn=2:4:3 (=2/9:4/9:3/9), and In:Ga:Zn=1:5:3 (=1/9:5/9:3/9); or ametal oxide having an atomic ratio which is close to any of the aboveatomic ratios can be used for the oxide semiconductor layer 408 a. Asthe oxide semiconductor layer 408 b, an In—Ga—Zn-based oxide having anyof atomic ratios of In:Ga:Zn=3:1:2 (=3/6:1/6:2/6), In:Ga:Zn=4:2:3(=4/9:2/9:3/9), In:Ga:Zn=5:1:3 (=5/9:1/9:3/9), In:Ga:Zn=5:3:4(=5/12:3/12:4/12), In:Ga:Zn=6:2:4 (=6/12:2/12:4/12), or In:Ga:Zn=7:1:3(=7/11:1/11:3/11); or a metal oxide having an atomic ratio which isclose to any of the above atomic ratios can be used.

Note that for example, the expression “the composition of an oxideincluding In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide containing In, Ga,and Zn at the atomic ratio, In: Ga: Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r². Forexample, r may be 0.05.

Note that the metal oxide which is applied to the oxide semiconductorstack 408 is not limited thereto, and a metal oxide which have anappropriate composition in accordance with required electricalcharacteristics (field-effect mobility, a threshold voltage, variations,and the like) may be used. In order to obtain the needed electricalcharacteristics, the carrier concentration, the impurity concentration,the defect density, the atomic ratio between a metal element and oxygen,the interatomic distance, the density, and the like are preferably setto appropriate values. For example, instead of gallium, another metalelement may be contained. Alternatively, a metal oxide which does notcontain zinc can be used. Note that it is preferable that a metal oxidewhich is to be applied contain zinc because a c-axis aligned crystallineoxide semiconductor (CAAC-OS) film can be relatively easily formed asthe oxide semiconductor layer that is formed later.

Note that the oxide semiconductor layer can be formed by a sputteringmethod. Generation of particles in deposition can be prevented by usinga sputtering target which contains indium. Therefore, it is preferablethat the oxide semiconductor layer 408 a and the oxide semiconductorlayer 408 b contain indium.

A structure of an oxide semiconductor layer is described below.

An oxide semiconductor layer is classified roughly into a single-crystaloxide semiconductor layer and a non-single-crystal oxide semiconductorlayer. The non-single-crystal oxide semiconductor layer includes any ofan amorphous oxide semiconductor layer, a microcrystalline oxidesemiconductor layer, a polycrystalline oxide semiconductor layer, ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, and thelike.

The amorphous oxide semiconductor layer has disordered atomicarrangement and no crystalline component. A typical example thereof isan oxide semiconductor layer in which no crystal part exists even in amicroscopic region, and the whole of the film is amorphous.

The microcrystalline oxide semiconductor layer includes a microcrystal(also referred to as nanocrystal) with a size greater than or equal to 1nm and less than 10 nm, for example. Thus, the microcrystalline oxidesemiconductor layer has a higher degree of atomic order than theamorphous oxide semiconductor layer. Hence, the density of defect statesof the microcrystalline oxide semiconductor layer is lower than that ofthe amorphous oxide semiconductor layer.

The CAAC-OS film is one of oxide semiconductor layers including aplurality of crystal parts, and most of the crystal parts each fitinside a cube whose one side is less than 100 nm. Thus, there is a casewhere a crystal part included in the CAAC-OS film fits inside a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.The density of defect states of the CAAC-OS film is lower than that ofthe microcrystalline oxide semiconductor layer. The CAAC-OS film isdescribed in detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor layer of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS film is changed by etching or the like, the c-axis might notbe necessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

In a transistor using the CAAC-OS film, change in electriccharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

Note that an oxide semiconductor layer may be a stacked film includingtwo or more of an amorphous oxide semiconductor layer, amicrocrystalline oxide semiconductor layer, and a CAAC-OS film, forexample.

For example, the CAAC-OS film is formed by a sputtering method with apolycrystalline metal oxide target. When ions collide with thesputtering target, a crystal region included in the sputtering targetmay be separated from the target along an a-b plane; in other words, asputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingtheir crystal state, whereby the CAAC-OS film can be formed.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle is attached to a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the metal oxide target for sputtering, an In—Ga—Zn—Ocompound target is described below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined ratio, applying pressure, and performing heat treatment ata temperature higher than or equal to 1000° C. and lower than or equalto 1500° C. Note that X, Y, and Z are each a given positive number.Here, the predetermined molar ratio of InO_(X) powder to GaO_(Y) powderand ZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3,or 3:1:2. The kinds of powder and the molar ratio for mixing powder maybe determined as appropriate depending on the desired sputtering target.

Note that the oxide semiconductor layer 408 a may have differentcrystallinity from the oxide semiconductor layer 408 b. Note that theCAAC-OS film is preferably applied to the oxide semiconductor layer 408b functioning as the channel of the transistor 300. In the case wherethe oxide semiconductor layer 408 b is formed using the CAAC-OS film, aprecursor might be oriented in the oxide semiconductor layer 408 aprovided on and in contact with the oxide semiconductor layer 408 b,i.e., the precursor might have what is called orderliness, so that theoxide semiconductor layer 408 a can be the CAAC-OS film. When the oxidesemiconductor layer 408 a provided on the back channel side is formed ofan amorphous oxide semiconductor, oxygen vacancies are generated inetching treatment for forming the source electrode layer 410 a and thedrain electrode layer 410 b, and accordingly the oxide semiconductorlayer 408 a is likely to be n-type. Therefore, it is preferable to applya crystalline oxide semiconductor to the oxide semiconductor layer 408a.

Note that in this embodiment, the oxide semiconductor layer 408 a andthe oxide semiconductor layer 408 b have the same constituent elementsand form an oxide semiconductor stack. At this time, depending onmaterials or deposition conditions of the oxide semiconductor layers,the interfaces between the oxide semiconductor layers are unclear insome cases. Therefore, in FIGS. 1A to 1C, the interface between theoxide semiconductor layer 408 a and the oxide semiconductor layer 408 bis schematically denoted by a dotted line. The same applies to drawingsmentioned below.

FIGS. 2A to 2C illustrate a structural example of a transistor 310. FIG.2A is a plan view of the transistor 310, FIG. 2B is a cross-sectionalview taken along a chain line X2-Y2 in FIG. 2A, and FIG. 2C is across-sectional view taken along a chain line V2-W2 in FIG. 2A. In amanner similar to the transistor 300 in FIGS. 1A to 1C, the transistor310 in FIGS. 2A to 2C includes the gate electrode layer 402 providedover the substrate 400 having an insulating surface, the gate insulatinglayer 404 over the gate electrode layer 402, the oxide semiconductorstack 408 which is in contact with the gate insulating layer 404 andoverlaps with the gate electrode layer 402, and the source electrodelayer 410 a and the drain electrode layer 410 b which are electricallyconnected to the oxide semiconductor stack 408. Further, the insulatinglayer 412 which covers the source electrode layer 410 a and the drainelectrode layer 410 b and is in contact with the oxide semiconductorstack 408 may be included as a component of the transistor 310.

The transistor 310 is different from the transistor 300 in that an oxidesemiconductor layer 408 c is provided between the oxide semiconductorlayer 408 b and the gate insulating layer 404. That is, in thetransistor 310, the oxide semiconductor stack 408 includes a stackedstructure of the oxide semiconductor layer 408 a, the oxidesemiconductor layer 408 b, and the oxide semiconductor layer 408 c.

Note that the transistor 310 has the same structure as the transistor300 except the oxide semiconductor layer 408 c; thus the description ofthe transistor 300 can be referred to.

By including the oxide semiconductor layer 408 c between the gateinsulating layer 404 and the oxide semiconductor layer 408 b in which achannel is formed, the transistor 310 has a structure in which carriersflow through a region which is apart from the gate insulating layer 404(what is called a buried channel). Accordingly, an interface between thegate insulating layer 404 and the channel can be stabilized andformation of a trap level at the interface can be prevented. Thus,degradation, in particular, photodegradation such as negative-biastemperature stress photodegradation of the transistor is prevented, sothat the transistor can have high reliability.

In a metal oxide containing indium and gallium, as the ratio of thegallium content to the content of the other metal element becomeshigher, the energy gap of the metal oxide is increased. In addition,owing to a difference in band gap between the oxide semiconductor layer408 c and the oxide semiconductor layer 408 b, a conduction band offsetis formed. Accordingly, it is preferable to use, as the oxidesemiconductor layer 408 c, a metal oxide in which the indium content islower than or equal to the gallium content because the channel can beeffectively formed in the oxide semiconductor layer 408 b.

In the case where an In—Ga—Zn-based oxide semiconductor is used as theoxide semiconductor layer 408 c, an In—Ga—Zn-based oxide having any ofatomic ratios of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=1:3:2(=1/6:3/6:2/6), In:Ga:Zn=2:4:3 (=2/9:4/9:3/9), or In:Ga:Zn=1:5:3(=1/9:5/9:3/9); or an oxide having an atomic ratio which is close to anyof the above atomic ratios can be used. Note it is preferable to use ametal oxide in which the indium content is lower than or equal to thegallium content for both the oxide semiconductor layer 408 a and theoxide semiconductor layer 408 c. Here, the composition of the oxidesemiconductor layer 408 a may be either different from or the same asthe composition of the oxide semiconductor layer 408 c.

In general, an oxide semiconductor layer is mostly deposited by asputtering method. On the other hand, when the oxide semiconductor layeris formed by sputtering, in some cases, an ionized rare gas element(e.g., argon) or a particle ejected from a surface of a sputteringtarget flicks off a particle of a film, such as a gate insulating film,on which the oxide semiconductor layer is to be formed. The particleflicked off from the film on which the oxide semiconductor layer is tobe formed might enter the oxide semiconductor layer and functions as animpurity element therein. In particular, a portion of the oxidesemiconductor layer, which is in the vicinity of the surface on whichthe oxide semiconductor layer is formed, might have high concentrationof the impurity element. Further, when the impurity element remains inthe vicinity of the surface where the oxide semiconductor layer is to beformed, the resistance of the oxide semiconductor layer is increased,which causes the electrical characteristics of the transistor to belowered.

However, in the transistor 310 illustrated in FIGS. 2A to 2C, since theoxide semiconductor layer 408 c is provided between the gate insulatinglayer 404 and the oxide semiconductor layer 408 b in which the channelis formed, a constituent element of the gate insulating layer 404 can beprevented from diffusing to the channel. That is, the oxidesemiconductor layer 408 c may contain the constituent element (e.g.,silicon) of the gate insulating layer 404 as an impurity. By includingthe oxide semiconductor layer 408 c, the transistor 310 can have morestabilized electrical characteristics; thus, a highly reliablesemiconductor device can be provided.

FIG. 2D is an energy band diagram in the thickness direction (D-D′) inFIG. 2B. In this embodiment, the oxide semiconductor layers 408 a to 408c are formed using In—Ga—Zn-based oxide semiconductors. Metal oxides inwhich the indium content is lower than or equal to the gallium contentare used for the oxide semiconductor layer 408 a and the oxidesemiconductor layer 408 c, and a metal oxide in which the indium contentis higher than the gallium content is used for the oxide semiconductorlayer 408 b. Accordingly, the oxide semiconductor stack 408 described inthis embodiment can have what is called a buried channel as shown in theenergy band of FIG. 2D.

An example of a method for manufacturing the transistor 310 is describedbelow with reference to FIGS. 3A to 3D.

First, the gate electrode layer 402 (including a wiring formed with thesame layer) is formed over the substrate 400 having an insulatingsurface.

There is no particular limitation on the substrate that can be used asthe substrate 400 having an insulating surface as long as it has heatresistance high enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. Alternatively, a singlecrystal semiconductor substrate or a polycrystalline semiconductorsubstrate made of silicon, silicon carbide, or the like, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like can be used. Furthermore, any of these substratesprovided with a semiconductor element may be used as the substrate 400.Further alternatively, a base insulating layer may be formed over thesubstrate 400.

The gate electrode layer 402 can be formed using a metal material suchas molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium,neodymium, or scandium, or an alloy material containing any of thesematerials as a main component. Alternatively, a semiconductor filmtypified by a polycrystalline silicon film doped with an impurityelement such as phosphorus, or a silicide film such as a nickel silicidefilm may be used as the gate electrode layer 402. The gate electrodelayer 402 may have a single-layer structure or a stacked-layerstructure. The gate electrode layer 402 may have a tapered shape with ataper angle of greater than or equal to 15° and less than or equal to70° for example. Here, the taper angle refers to an angle formed betweena side surface of a layer having a tapered shape and a bottom surface ofthe layer.

The material of the gate electrode layer 402 may be a conductivematerial such as indium oxide-tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium oxide-zinc oxide, or indium tin oxide to which siliconoxide is added.

Alternatively, as the material of the gate electrode layer 402, anIn—Ga—Zn-based oxide containing nitrogen, an In—Sn-based oxidecontaining nitrogen, an In—Ga-based oxide containing nitrogen, anIn—Zn-based oxide containing nitrogen, an Sn-based oxide containingnitrogen, an In-based oxide containing nitrogen, or a metal nitride film(such as an indium nitride film, a zinc nitride film, a tantalum nitridefilm, or a tungsten nitride film) may be used. These materials have awork function of 5 eV or more. Therefore, when the gate electrode layer402 is formed using any of these materials, the threshold voltage of thetransistor can be positive, so that the transistor can be a normally-offswitching transistor.

Next, the gate insulating layer 404 is formed so as to cover the gateelectrode layer 402 (see FIG. 3A). As the gate insulating layer 404, asingle layer or a stack of layers including at least one of thefollowing films formed by a plasma CVD method, a sputtering method, orthe like is used: a silicon oxide film, a silicon oxynitride film, asilicon nitride oxide film, a silicon nitride film, an aluminum oxidefilm, a hafnium oxide film, an yttrium oxide film, a zirconium oxidefilm, a gallium oxide film, a tantalum oxide film, a magnesium oxidefilm, a lanthanum oxide film, a cerium oxide film, and a neodymium oxidefilm.

Note that it is preferable that a region which is included in the gateinsulating layer 404 and is in contact with the oxide semiconductorstack 408 formed later be formed using an oxide insulating layer and itis further preferable that the region include an oxygen-excess region.In order to provide the oxygen-excess region in the gate insulatinglayer 404, the gate insulating layer 404 may be formed in an oxygenatmosphere, for example. Alternatively, oxygen may be introduced intothe deposited gate insulating layer 404 to provide the oxygen-excessregion. As a method for introducing oxygen, an ion implantation method,an ion doping method, a plasma immersion ion implantation method, plasmatreatment, or the like may be used.

Next, an oxide semiconductor film to be the oxide semiconductor layer408 c, an oxide semiconductor film to be the oxide semiconductor layer408 b, and an oxide semiconductor film to be the oxide semiconductorlayer 408 a are deposited in this order over the gate insulating layer404 and processed into an island shape by etching treatment using aphotolithography method to form the oxide semiconductor stack 408 (seeFIG. 3B).

The oxide semiconductor layer 408 c, the oxide semiconductor layer 408b, and the oxide semiconductor layer 408 a may be each an amorphousoxide semiconductor or a crystalline oxide semiconductor. However, it ispreferable that the oxide semiconductor layer 408 b functioning as thechannel of the transistor 310 be a crystalline oxide semiconductor. Thecrystalline oxide semiconductor may be obtained by performing heattreatment on an amorphous oxide semiconductor. The heat treatment forcrystallizing the amorphous oxide semiconductor is performed at atemperature higher than or equal to 250° C. and lower than or equal to700° C., preferably higher than or equal to 400° C., further preferablyhigher than or equal to 500° C., still further preferably higher than orequal to 550° C. Note that the heat treatment can also serve as anotherheat treatment in the manufacturing process.

The oxide semiconductor films each can be formed by a sputtering method,a molecular beam epitaxy (MBE) method, a CVD method, a pulse laserdeposition method, an atomic layer deposition (ALD) method, or the likeas appropriate.

In the formation of the oxide semiconductor films, the hydrogenconcentration in the oxide semiconductor films is preferably reduced asmuch as possible. In order to reduce the hydrogen concentration, forexample, in the case where a sputtering method is used for thedeposition, a high-purity rare gas (typically, argon) from whichimpurities such as hydrogen, water, a hydroxyl group, or a hydride havebeen removed; oxygen; or a mixed gas of oxygen and the rare gas is usedas appropriate as an atmosphere gas supplied to a deposition chamber ofa sputtering apparatus.

The oxide semiconductor films are formed in such a manner that asputtering gas from which hydrogen and moisture have been removed isintroduced into the deposition chamber while moisture remaining thereinis removed, whereby the hydrogen concentration in the formed oxidesemiconductor films can be reduced. In order to remove the residualmoisture in the deposition chamber, an entrapment vacuum pump, forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo molecular pumpprovided with a cold trap. A cryopump has a high capability in removinga hydrogen molecule, a compound containing a hydrogen atom such as water(H₂O) (preferably, also a compound containing a carbon atom), and thelike; therefore, the impurity concentration in the oxide semiconductorfilm formed in the deposition chamber which is evacuated using acryopump can be reduced.

The gate insulating layer 404 and the oxide semiconductor films arepreferably formed in succession without exposure to the air. By formingthe gate insulating layer 404 and the oxide semiconductor films insuccession without exposure to the air, attachment of hydrogen or ahydrogen compound (e.g., adsorption water) onto surfaces of the oxidesemiconductor films can be prevented, and thus mixing of an impurity canbe prevented.

Further, in the case where the oxide semiconductor film is formed by asputtering method, the relative density (the fill rate) of a metal oxidetarget which is used for forming the oxide semiconductor film is greaterthan or equal to 90% and less than or equal to 100%, preferably greaterthan or equal to 95% and less than or equal to 99.9%. With the use of ametal oxide target with a high relative density, a dense oxidesemiconductor film can be deposited.

Note that formation of the oxide semiconductor film while the substrate400 is kept at high temperature is also effective in reducing theimpurity concentration in the oxide semiconductor film. The temperatureat which the substrate 400 is heated may be higher than or equal to 150°C. and lower than or equal to 450° C.; the substrate temperature ispreferably higher than or equal to 200° C. and lower than or equal to350° C. By heating the substrate at high temperature during the filmformation, a crystalline oxide semiconductor film can be formed.

In the case where a CAAC-OS film is employed as the oxide semiconductorlayer (e.g., the oxide semiconductor layer 408 b), any of the followingmethods can be used for forming the CAAC-OS film, for example. One ofthe methods is to form an oxide semiconductor film at a depositiontemperature higher than or equal to 200° C. and lower than or equal to450° C., thereby obtaining c-axis alignment substantially perpendicularto a surface. Another method is to form a thin oxide semiconductor filmand then subject the film to heat treatment at a temperature higher thanor equal to 200° C. and lower than or equal to 700° C., therebyobtaining c-axis alignment substantially perpendicular to a surface. Theother method is to form a first thin oxide semiconductor film, heattreatment is performed on the film at a temperature higher than or equalto 200° C. and lower than or equal to 700° C., and then form a secondoxide semiconductor film, thereby obtaining c-axis alignmentsubstantially perpendicular to a surface.

Oxide semiconductors used for the oxide semiconductor layers 408 a to408 c contain at least indium (In). In particular, indium and zinc (Zn)are preferably contained. As a stabilizer for reducing variation inelectric characteristics of a transistor using the oxide semiconductor,gallium (Ga) is preferably additionally contained.

Further, as described above, it is preferable that the oxidesemiconductor layer 408 a and the oxide semiconductor layer 408 cfunctioning buffer layers for reducing influence of a trap level be eachformed using a metal oxide in which the indium content is lower than orequal to the gallium content, and the oxide semiconductor layer 408 bserving as a channel formation region be formed using a metal oxide inwhich the indium content is higher than the gallium content.

As a stabilizer, one or more of tin (Sn), hafnium (Hf), aluminum (Al),and zirconium (Zr) may be contained instead of gallium (Ga) or inaddition to gallium (Ga). As another stabilizer, one or plural kinds oflanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), or lutetium (Lu) may be contained.

As the oxide semiconductor, for example, any of the following can beused: a single-component metal oxide such as indium oxide, tin oxide, orzinc oxide; a two-component metal oxide such as an In—Zn-based oxide, anIn—Mg-based oxide, or an In—Ga-based oxide; a three-component metaloxide such as an In—Ga—Zn-based oxide, an In—Al—Zn-based oxide, anIn—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide,an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-basedoxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, anIn—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide,an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-basedoxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; and afour-component metal oxide such as an In—Sn—Ga—Zn-based oxide, anIn—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn-Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or anIn—Hf—Al—Zn-based oxide.

For example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga,and Zn as its main components and there is no particular limitation onthe ratio of In, Ga, and Zn. The In—Ga—Zn-based oxide may contain ametal element other than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 issatisfied, and m is not an integer) may be used as an oxidesemiconductor. Note that M represents one or more metal elementsselected from Ga, Fe, Mn, and Co. Alternatively, as the oxidesemiconductor, a material expressed by a chemical formula,In₂SnO₅(ZnO)_(n) (n>0, n is a natural number) may be used.

Further, the oxide semiconductor stack 408 is preferably subjected toheat treatment for removing excess hydrogen (including water and ahydroxyl group) contained in the films (for dehydration ordehydrogenation). The temperature of the heat treatment is higher thanor equal to 300° C. and lower than or equal to 700° C., or lower thanthe strain point of the substrate. The heat treatment can be performedunder reduced pressure, a nitrogen atmosphere, or the like. Hydrogen,which is an impurity imparting n-type conductivity, can be removed bythe heat treatment.

Note that the heat treatment for the dehydration or dehydrogenation maybe performed at any timing in the manufacturing process of thetransistor as long as it is performed after the formation of the oxidesemiconductor films. The heat treatment for dehydration ordehydrogenation may be performed a plurality of times, and may alsoserve as another heat treatment.

In the heat treatment, it is preferable that water, hydrogen, or thelike be not contained in nitrogen or a rare gas such as helium, neon, orargon. The purity of nitrogen or the rare gas such as helium, neon, orargon which is introduced into the heat treatment apparatus is set topreferably 6N (99.9999%) or higher, further preferably 7N (99.99999%) orhigher (that is, the impurity concentration is preferably 1 ppm orlower, further preferably 0.1 ppm or lower).

In addition, after the oxide semiconductor layers (or the oxidesemiconductor stack) are heated by the heat treatment, a high-purityoxygen gas, a high-purity dinitrogen gas, or ultra dry air (the moistureamount is less than or equal to 20 ppm (−55° C. by conversion into a dewpoint), preferably less than or equal to 1 ppm, further preferably lessthan or equal to 10 ppb, in the measurement with use of a dew pointmeter of a cavity ring down laser spectroscopy (CRDS) system) may beintroduced into the same furnace while the heating temperature is beingmaintained or being gradually decreased. It is preferable that water,hydrogen, or the like be not contained in the oxygen gas or thedinitrogen monoxide gas. The purity of the oxygen gas or the dinitrogenmonoxide gas which is introduced into the heat treatment apparatus ispreferably 6N or more, further preferably 7N or more (i.e., the impurityconcentration in the oxygen gas or the dinitrogen monoxide gas ispreferably 1 ppm or lower, further preferably 0.1 ppm or lower). Theoxygen gas or the dinitrogen monoxide gas acts to supply oxygen that isa main component of the oxide semiconductor and that has been reduced bythe step of removing an impurity for the dehydration or dehydrogenation,so that the oxide semiconductor layers can have high purity and bei-type (intrinsic) oxide semiconductor layers.

Since there is a possibility that oxygen is also released and reduced bydehydration or dehydrogenation treatment, oxygen (including at least oneof an oxygen radical, an oxygen atom, and an oxygen ion) may beintroduced into the oxide semiconductor layers which have been subjectedto the dehydration or dehydrogenation treatment to supply oxygen to thelayers.

Introduction (supply) of oxygen into the dehydrated or dehydrogenatedoxide semiconductor layers enables the oxide semiconductor layers to behighly purified and to be i-type (intrinsic). Variation in electriccharacteristics of a transistor having the highly-purified and i-type(intrinsic) oxide semiconductor is suppressed, and the transistor iselectrically stable.

In the case where oxygen is introduced into the oxide semiconductorlayers, oxygen may be directly introduced into the oxide semiconductorlayers or may be introduced into the oxide semiconductor layers throughan insulating layer formed in a later step. As the method for addingoxygen (containing at least one of an oxygen radical, an oxygen atom,and an oxygen ion), an ion implantation method, an ion doping method, aplasma immersion ion implantation method, plasma treatment, or the likecan be used. A gas containing oxygen can be used for oxygen introducingtreatment. As the gas containing oxygen, oxygen, dinitrogen monoxide,nitrogen dioxide, carbon dioxide, carbon monoxide, and the like can beused. Further, a rare gas may be contained in the gas containing oxygenin the oxygen introducing treatment.

For example, in the case where an oxygen ion is introduced into theoxide semiconductor layers by an ion implantation method, the dose canbe greater than or equal to 1×10¹³ ions/cm² and less than or equal to5×10¹⁶ ions/cm².

The timing of supply of oxygen to the oxide semiconductor layers is notparticularly limited to the above as long as it is after the formationof the oxide semiconductor films. Oxygen may be introduced into theoxide semiconductor layers plural times.

Then, a conductive film is formed over the oxide semiconductor stack 408and processed to form the source electrode layer 410 a and the drainelectrode layer 410 b (including a wiring formed with the same layer)(see FIG. 3C).

For the source electrode layer 410 a and the drain electrode layer 410b, a metal film containing an element selected from Al, Cr, Cu, Ta, Ti,Mo, and W, a metal nitride film containing any of the above elements asa component (a titanium nitride film, a molybdenum nitride film, or atungsten nitride film), or the like can be used, for example.Alternatively, a film of a high-melting-point metal such as Ti, Mo, or Wor a metal nitride film of any of these elements (a titanium nitridefilm, a molybdenum nitride film, or a tungsten nitride film) may bestacked on one of or both a bottom side and a top side of a metal filmof Al, Cu, or the like. Alternatively, the source electrode layer 410 aand the drain electrode layer 410 b may be formed using a conductivemetal oxide. As the conductive metal oxide, indium oxide (In₂O₃), tinoxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxide (In₂O₃—SnO₂),indium oxide-zinc oxide (In₂O₃—ZnO), or any of these metal oxidematerials in which silicon oxide is contained can be used.

For the source electrode layer 410 a and the drain electrode layer 410b, a metal nitride film such as an In—Ga—Zn—O film containing nitrogen,an In—Sn—O film containing nitrogen, an In—Ga—O film containingnitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containingnitrogen, or an InO film containing nitrogen can be used. These filmscontain the same constituent elements as the oxide semiconductor stack408 and can therefore stabilize the interface with the oxidesemiconductor stack 408.

Note that it is preferable to use a conductive film containing copperfor the source electrode layer 410 a and the drain electrode layer 410 bbecause wiring resistance can be reduced. In general, copper easilydiffuses into a semiconductor or a silicon oxide film, and thus theoperation of a semiconductor device might be unstable and yield might besignificantly reduced. However, in the transistor 310, since the oxidesemiconductor layer 408 a serving as a buffer layer for preventing theconstituent element (that is copper here) of the electrode layers isprovided in contact with the source electrode layer 410 a and the drainelectrode layer 410 b, influence of the trap level on the back channelside which is generated owing to diffusion of copper can be reduced,preferably can be eliminated.

Next, the insulating layer 412 is formed to cover the source electrodelayer 410 a, the drain electrode layer 410 b, and the exposed oxidesemiconductor stack 408 (see FIG. 3D).

The insulating layer 412 can be formed using a single layer or a stackof layers of one or more of the following films formed by a plasma CVDmethod or a sputtering method: a silicon oxide film, a gallium oxidefilm, an aluminum oxide film, a silicon nitride film, a siliconoxynitride film, an aluminum oxynitride film, a silicon nitride oxidefilm, and the like. However, it is preferable that an oxide insulatinglayer be formed as the insulating layer 412 because the oxide insulatinglayer can supply oxygen to the oxide semiconductor stack 408.

Heat treatment may be performed after the insulating layer 412 isformed. A temperature of the heat treatment is preferably higher than orequal to 200° C., and for example, 220° C.

Through the above steps, the transistor 310 of this embodiment can beformed.

The transistor of this embodiment includes the first oxide semiconductorlayer which serves as a buffer layer for preventing diffusion of theconstituent element of the source electrode layer and the drainelectrode layer into the channel and the second oxide semiconductorlayer which serves as the channel. With such a structure, influence ofan interface state which can be formed on the back channel side of thetransistor can be reduced. Further, it is preferable that the transistordescribed in this embodiment further include the third oxidesemiconductor layer which is provided between the oxide semiconductorlayer serving as the channel and the gate insulating layer and serves asa buffer layer for preventing degradation of an interface on the gateinsulating layer side of the channel. When the third oxide semiconductorlayer is provided in the transistor, photodegradation (e.g.,negative-bias temperature stress photodegradation) of the transistor canbe reduced, so that a highly reliable semiconductor device can beprovided.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 2

In this embodiment, an embodiment of a semiconductor device which isdifferent from that in Embodiment 1 will be described with reference toFIGS. 10A and 10B. Specifically, transistors each including a gateinsulating layer with a structure which is different from that of thetransistor of Embodiment 1 are described.

A structural example of a transistor 320 is illustrated in FIG. 10A. Ina manner similar to the transistor 310 in FIGS. 2A to 2D, the transistor320 in FIG. 10A includes the gate electrode layer 402 provided over thesubstrate 400 having an insulating surface, the gate insulating layer403 and 406 over the gate electrode layer 402, the oxide semiconductorstack 408 which is in contact with the gate insulating layer 406 andoverlaps with the gate electrode layer 402, and the source electrodelayer 410 a and the drain electrode layer 410 b which are electricallyconnected to the oxide semiconductor stack 408. In the transistor 320,the oxide semiconductor stack 408 includes the oxide semiconductor layer408 c in contact with the gate insulating layer 406, the oxidesemiconductor layer 408 b on and in contact with the oxide semiconductorlayer 408 c, and the oxide semiconductor layer 408 a on and in contactwith the oxide semiconductor layer 408 b and in contact with the sourceelectrode layer 410 a and the drain electrode layer 410 b. Further, theinsulating layer 412 which covers the source electrode layer 410 a andthe drain electrode layer 410 b and is in contact with the oxidesemiconductor stack 408 may be included as a component of the transistor320.

The transistor 320 is different from the transistor 310 in that the gateinsulating layer has a structure in which a first gate insulating layer403 which includes gate insulating layers 403 a, 403 b, and 403 c, and asecond gate insulating layer 406 are stacked from the gate electrodelayer 402 side.

Note that the transistor 320 has the same structure as the transistor310 except the gate insulating layer; therefore, the description of thetransistor 310 can be referred to.

In the transistor 320, a nitrogen-containing silicon film is applied tothe first gate insulating layer 403. A nitrogen-containing silicon filmhas higher relative permittivity than a silicon oxide film and needs tohave a larger thickness than a silicon oxide film to obtain the sameelectrostatic capacity; thus, it is possible to physically increase thethickness of the gate insulating layer. Accordingly, a reduction inwithstand voltage of the transistor 320 is prevented and further thewithstand voltage is improved, so that electrostatic breakdown of thesemiconductor device can be prevented.

Further, as the second gate insulating layer 406 in contact with theoxide semiconductor stack 408, an insulating layer containing oxygen,such as a silicon oxide film, a gallium oxide film, or an aluminum oxidefilm, is used. It is further preferable that the second gate insulatinglayer 406 include a region which contains oxygen in a proportion higherthan that of oxygen in the stoichiometric composition (i.e., anoxygen-excess region). This is because when the insulating layer incontact with the oxide semiconductor stack 408 includes an oxygen-excessregion, oxygen can be supplied to the oxide semiconductor stack 408 andthus oxygen can be prevented from being released from the oxidesemiconductor stack 408, and oxygen vacancies in the oxide semiconductorstack 408 can be filled. In order to provide the oxygen-excess region inthe second gate insulating layer 406, the second gate insulating layer406 is formed in an oxygen atmosphere, for example. Alternatively,oxygen may be introduced into the deposited second gate insulating layer406 to provide the oxygen-excess region therein.

A silicon nitride film, a silicon nitride oxide film, and a siliconoxynitride film can be given as examples of the nitrogen-containingsilicon film which is applied to the first gate insulating layer 403.However, as the ratio of the nitrogen content to the oxygen contentbecomes higher, relative permittivity is increased and thus a siliconnitride film is preferably used. Further, the energy gap of siliconnitride is as small as 5.5 eV whereas the energy gap of silicon oxide is8 eV, and the specific resistance of silicon nitride is smallaccordingly; therefore, use of a silicon nitride film can lead to highelectrostatic discharge (ESD) resistance. Note that in thisspecification, a “silicon oxynitride film” refers to a film thatincludes more oxygen than nitrogen, and a “silicon nitride oxide film”refers to a film that includes more nitrogen than oxygen.

The gate insulating layer 403 a in contact with the gate electrode layer402 is a silicon film which contains a smaller amount of ammonia than atleast the gate insulating layer 403 b. Ammonia serves as a ligand of ametal complex owing to a function of a lone electron-pair on a nitrogenatom. Therefore, for example, in the case where copper is used for thegate electrode layer 402, when a gate insulating layer containing alarge amount of ammonia is provided in contact with the gate electrodelayer, the copper might diffuse into the gate insulating layer owing toreaction represented by Formula (1) described below.

[Formula 1]

Cu²⁺+4NH₃

[Cu(NH₃)₄]²⁺  (1)

When the gate insulating layer 403 a containing a small amount ofammonia (a smaller amount ammonia than at least the gate insulatinglayer 403 b) is provided in contact with the gate electrode layer 402 inthe transistor 320, a material (e.g., copper) of the gate electrodelayer 402 can be prevented from diffusing into the first gate insulatinglayer 403. In other words, the gate insulating layer 403 a can functionas a barrier film against a metal material included in the gateelectrode layer 402. By including the gate insulating layer 403 a, thetransistor can have higher reliability.

As the gate insulating layer 403 b, a nitrogen-containing silicon filmwhich has a larger thickness than the gate insulating layer 403 a and inwhich the number of defects is reduced is used. For example, the gateinsulating layer 403 b has a thickness greater than or equal to 300 nmand less than or equal to 400 nm. Further, in the nitrogen-containingsilicon film to be used as the gate insulating layer 403 b, the spindensity of a signal which appears in the Nc center (the g value is2.003) of signals measured by electron spin resonance (ESR) ispreferably less than or equal to 1×10¹⁷ spins/cm³ and further preferablyless than or equal to 5×10¹⁶ spins/cm³. As described above, by providinga nitrogen-containing silicon film having a large thickness (e.g., 300nm or more) in which the number of defects is reduced, it is possiblethat the gate insulating layer 403 b has an ESD resistance of 300 V ormore.

As the gate insulating layer 403 c, a nitrogen-containing silicon filmin which the hydrogen concentration is reduced is used. The hydrogenconcentration of the gate insulating layer 403 c is lower than at leastthat of the gate insulating layer 403 b. For example, in the case wherethe gate insulating layer 403 c is deposited by a plasma CVD method, thehydrogen concentration contained in a supply gas is reduced to lowerthan the hydrogen concentration of a supply gas used for depositing thegate insulating layer 403 b, whereby the hydrogen concentration of thegate insulating layer 403 c can be lower than that of gate insulatinglayer 403 b. Specifically, in the case where silicon nitride films areformed as the gate insulating layer 403 b and the gate insulating layer403 c, the gate insulating layer 403 c may be deposited using a supplygas containing a smaller amount of ammonia than a supply gas fordepositing the gate insulating layer 403 b or may be deposited withoutusing ammonia.

When a silicon nitride film in which the hydrogen concentration isreduced is provided as the gate insulating layer 403 c, the amount ofhydrogen or a hydrogen compound (e.g., water) which is mixed into thesecond gate insulating layer 406 and the oxide semiconductor stack 408can be reduced. Part of hydrogen serves as a donor by being bonded to anoxide semiconductor and electrons serving as carriers are generated,which causes the threshold voltage of the transistor to be changed(shifted) in the negative direction. Accordingly, by including a siliconnitride film in which the hydrogen concentration is reduced as the gateinsulating layer 403 c, the transistor can have stabilized electricalcharacteristics. Further, by being provided as the gate insulating layer403 c, the silicon nitride film in which the hydrogen concentration isreduced has an effect as a barrier film for preventing impurities suchas hydrogen and a hydrogen compound included in the gate insulatinglayer 403 b from diffusing into the oxide semiconductor stack 408.

In this embodiment, silicon nitride films are used as the gateinsulating layer 403 a, the gate insulating layer 403 b, and the gateinsulating layer 403 c which are included in the first gate insulatinglayer 403; and a silicon oxynitride film is used as the second gateinsulating layer 406. The gate insulating layers are formed successivelyby a plasma CVD method. Specifically, a mixed gas of silane (SiH₄) andnitrogen (N₂) is supplied and a silicon nitride film to be the gateinsulating layer 403 a is deposited; the supply gas is switched to amixed gas of silane (SiH₄), nitrogen (N₂), and ammonia (NH₃) and asilicon nitride film to be the gate insulating layer 403 b is deposited;then the supply gas is switched to a mixed gas of silane (SiH₄) andnitrogen (N₂) and a silicon nitride film to be the gate insulating layer403 c is deposited; and then the supply gas is switched to a mixed gasof silane (SiH₄) and dinitrogen monoxide (N₂O) and a silicon oxynitridefilm to be the second gate insulating layer 406 is deposited.

The gate insulating layer 403 a preferably has a thickness greater thanor equal to 30 nm and less than or equal to 100 nm and furtherpreferably has a thickness greater than or equal to 30 nm and less thanor equal to 50 nm. The gate insulating layer 403 b, which is provided asa countermeasure against electrostatic breakdown of the transistor,preferably has a thickness greater than or equal to 300 nm and less thanor equal to 400 nm. The gate insulating layer 403 c, which functions asa barrier film for preventing diffusion of hydrogen into the oxidesemiconductor stack 408, preferably has a thickness greater than orequal to 25 nm and less than or equal to 150 nm. The second gateinsulating layer 406 preferably has a thickness greater than or equal to25 nm and less than or equal to 100 nm. Note that the thickness of eachof the gate insulating layers is preferably adjusted so that the totalof the thickness of the first gate insulating layer 403 (the totalthickness of the gate insulating layer 403 a, the gate insulating layer403 b, and the gate insulating layer 403 c) and the thickness of thesecond gate insulating layer 406 is greater than or equal to 355 nm andless than or equal to 550 nm.

Note that as in a transistor 330 illustrated in FIG. 10B, the gateinsulating layer formed of a stack of the first gate insulating layer403 and the second gate insulating layer 406 and the oxide semiconductorstack 408 formed of a stack of the oxide semiconductor layer 408 b andthe oxide semiconductor layer 408 a can be used in combination.

The transistor of this embodiment includes the first oxide semiconductorlayer which serves as a buffer layer for preventing diffusion of theconstituent element of the source electrode layer and the drainelectrode layer into the channel and the second oxide semiconductorlayer which serves as the channel. With such a structure, influence ofan interface state which can be formed on the back channel side of thetransistor can be reduced. Further, it is preferable that the transistordescribed in this embodiment further include the third oxidesemiconductor layer which is provided between the oxide semiconductorlayer serving as the channel and the gate insulating layer and serves asa buffer layer for preventing degradation of an interface on the gateinsulating layer side of the channel. When the third oxide semiconductorlayer is provided in the transistor, photodegradation (e.g.,negative-bias temperature stress photodegradation) of the transistor canbe reduced, so that a highly reliable semiconductor device can beprovided.

Further, the transistor of this embodiment has, as the gate insulatinglayer, a stacked structure which is formed of a first gate insulatinglayer including a nitrogen-containing silicon film functioning as abarrier film against a constituent element (e.g., copper) of a gateelectrode layer, a nitrogen-containing silicon film having a largethickness (e.g., 300 nm) in which the number of defects is reduced, anda nitrogen-containing silicon film having a blocking property againsthydrogen; and a second gate insulating layer containing oxygen.Accordingly, in the transistor of this embodiment, variations in theelectrical characteristics and electrostatic breakdown are prevented. Byincluding such a transistor, a semiconductor device can have highreliability, and the semiconductor device can be provided in a highyield.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 3

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using the transistor described inEmbodiment 1 or 2. Further, part or all of the driver circuitry whichincludes the transistor can be formed over a substrate where a pixelportion is formed, whereby a system-on-panel can be formed.

In FIG. 4A, a sealant 4005 is provided so as to surround a pixel portion4002 provided over a substrate 4001, and the pixel portion 4002 issealed with a substrate 4006. In FIG. 4A, a scan line driver circuit4004 and a signal line driver circuit 4003 which are each formed using asingle crystal semiconductor film or a polycrystalline semiconductorfilm over an IC chip or a substrate separately prepared are mounted onthe substrate 4001, in a region that is different from the regionsurrounded by the sealant 4005. Various signals and potentials which areprovided to the pixel portion 4002 through the signal line drivercircuit 4003 and the scan line driver circuit 4004 are supplied fromflexible printed circuits (FPCs) 4018 a and 4018 b.

In FIGS. 4B and 4C, the sealant 4005 is provided to surround the pixelportion 4002 and the scan line driver circuit 4004 which are providedover the substrate 4001. The substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Consequently, thepixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with a display element by the substrate 4001, the sealant 4005,and the substrate 4006. In FIGS. 4B and 4C, the signal line drivercircuit 4003 which is formed using a single crystal semiconductor filmor a polycrystalline semiconductor film over an IC chip or a substrateseparately prepared is mounted on the substrate 4001, in a region thatis different from the region surrounded by the sealant 4005. In FIGS. 4Band 4C, various signals and potentials which are provided to the pixelportion 4002 through the signal line driver circuit 4003 and the scanline driver circuit 4004 are supplied from an FPC 4018.

Although FIGS. 4B and 4C each illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on thesubstrate 4001, one embodiment of the present invention is not limitedto this structure. The scan line driver circuit may be separately formedand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be separately formed and thenmounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method or the like can beused. FIG. 4A shows an example in which the signal line driver circuit4003 and the scan line driver circuit 4004 are mounted by a COG method.FIG. 4B shows an example in which the signal line driver circuit 4003 ismounted by a COG method. FIG. 4C shows an example in which the signalline driver circuit 4003 is mounted by a TAB method.

Note that the display device includes a panel in which the displayelement is sealed, and a module in which an IC including a controller orthe like is mounted on the panel. Specifically, a display device in thisspecification means an image display device, a display device, or alight source (including a lighting device). Furthermore, the displaydevice also includes the following modules in its category: a module towhich a connector such as an FPC or a TCP is attached; a module having aTCP at the tip of which a printed wiring board is provided; and a modulein which an integrated circuit (IC) is directly mounted on a displayelement by a COG method.

The pixel portion and the scan line driver circuit provided over thesubstrate include a plurality of transistors, and the transistordescribed in Embodiment 1 or 2 can be applied thereto.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes an elementwith luminance which is controlled by current or voltage, in itscategory, and specifically includes an inorganic electroluminescent (EL)element, an organic EL element, and the like. Furthermore, a displaymedium whose contrast is changed by an electric effect, such as anelectronic ink display device (electronic paper), can be used.

Embodiments of the semiconductor device are described with reference toFIGS. 4A to 4C and FIG. 5. FIG. 5 corresponds to a cross-sectional viewalong a line M-N in FIG. 4B. An example of a liquid crystal displaydevice using a liquid crystal element as a display element isillustrated in FIG. 5.

Note that there is no limitation on the display device as long as thetransistor 4010 provided in the pixel portion 4002 is electricallyconnected to the display element and the display element can performdisplay, and a variety of display elements can be used.

As illustrated in FIGS. 4A to 4C and FIG. 5, the semiconductor deviceincludes a connection terminal electrode 4015 and a terminal electrode4016, and the connection terminal electrode 4015 and the terminalelectrode 4016 are electrically connected to a terminal included in theFPC 4018 (4018 a or 4018 b) through an anisotropic conductive layer4019.

The connection terminal electrode 4015 is formed from the sameconductive layer as a first electrode layer 4034. The terminal electrode4016 is formed from the same conductive layer as a source electrodelayer and a drain electrode layer of the transistor 4010 and atransistor 4011.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the substrate 4001 include a plurality of transistors. FIGS. 4A to4C and FIG. 5 illustrate the transistor 4010 included in the pixelportion 4002 and the transistor 4011 included in the scan line drivercircuit 4004. In FIG. 5, an insulating layer 4032 is provided over thetransistors 4010 and 4011.

The transistor described in Embodiment 1 or 2 can be applied to thetransistor 4010 and the transistor 4011. In this embodiment, an examplein which a transistor having a structure similar to that of thetransistor 310 described in Embodiment 1 is used is described. Thetransistors 4010 and 4011 are bottom-gate transistors.

Each of the transistors 4010 and 4011 includes a first oxidesemiconductor layer which functions as a buffer layer for preventingdiffusion of a constituent element of the source and drain electrodelayers in to a channel, a second oxide semiconductor layer whichfunctions as the channel, and a third oxide semiconductor layer which isprovided between a gate insulating layer and the oxide semiconductorlayer functioning as the channel and functions as a buffer layer forpreventing degradation of the interface on the gate insulating layerside of the channel. Accordingly, the transistors 4010 and 4011 are eacha highly reliable transistor in which influence of an interface statewhich can be formed on the back channel side is reduced andphotodegradation (e.g., negative-bias temperature stressphotodegradation) is reduced.

Moreover, a conductive layer may be provided so as to overlap with achannel formation region of the oxide semiconductor layer of thetransistor 4011 for the driver circuit. When the conductive layer isprovided so as to overlap with the channel formation region of the oxidesemiconductor layer, the amount of change in the threshold voltage ofthe transistor 4011 can be further reduced. The conductive layer mayhave the same potential as or a potential different from that of a gateelectrode layer of the transistor 4011, and can function as a secondgate electrode layer. Alternatively, the potential of the conductivelayer may be in a floating state.

The conductive layer also functions to block an external electric field,that is, to prevent an external electric field (particularly, to preventstatic electricity) from effecting the inside (a circuit portionincluding a transistor). A blocking function of the conductive layer canprevent the variation in electrical characteristics of the transistordue to the effect of external electric field such as static electricity.

In FIG. 5, a liquid crystal element 4013 includes a first electrodelayer 4034, a second electrode layer 4031, and a liquid crystal layer4008. Note that insulating layers 4033 and 4038 functioning as alignmentfilms are provided so that the liquid crystal layer 4008 is positionedtherebetween. The second electrode layer 4031 is provided on thesubstrate 4006 side, and the first electrode layer 4034 and the secondelectrode layer 4031 are stacked with the liquid crystal layer 4008provided therebetween.

The first electrode layer 4034 and the second electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide, indium zinc oxide, indiumtin oxide to which silicon oxide is added, or graphene.

Alternatively, the first electrode layer 4034 and the second electrodelayer 4031 can be formed using one or more materials selected frommetals such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium(Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt(Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper(Cu), and silver (Ag); an alloy of any of these metals; and a nitride ofany of these metals.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the first electrodelayer 4034 and the second electrode layer 4031. As the conductive highmolecule, what is called a π-electron conjugated conductive polymer canbe used. For example, polyaniline or a derivative thereof, polypyrroleor a derivative thereof, polythiophene or a derivative thereof, acopolymer of two or more of aniline, pyrrole, and thiophene or aderivative thereof can be given.

A spacer 4035 is a columnar spacer obtained by selective etching of aninsulating layer and is provided in order to control the thickness ofthe liquid crystal layer 4008 (a cell gap). Alternatively, a sphericalspacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a ferroelectric liquid crystal,an anti-ferroelectric liquid crystal, or the like can be used. Theseliquid crystal materials may be a low molecular compound or a highmolecular compound. Such a liquid crystal material (liquid crystalcomposition) exhibits a cholesteric phase, a smectic phase, a cubicphase, a chiral nematic phase, an isotropic phase, or the like dependingon conditions.

Alternatively, a liquid crystal composition exhibiting a blue phase forwhich an alignment film is unnecessary may be used for the liquidcrystal layer 4008. In this case, the liquid crystal layer 4008 is incontact with the first electrode layer 4034 and the second electrodelayer 4031. A blue phase is one of liquid crystal phases, which isgenerated just before a cholesteric phase changes into an isotropicphase while temperature of cholesteric liquid crystal is increased. Theblue phase can be exhibited using a liquid crystal composition which isa mixture of a liquid crystal and a material. In order to increase thetemperature range where the blue phase is exhibited, a liquid crystallayer may be formed by adding a polymerizable monomer, a polymerizationinitiator, and the like to a liquid crystal composition exhibiting ablue phase and by performing polymer stabilization treatment. The liquidcrystal composition exhibiting a blue phase has a short response time,and has optical isotropy, which contributes to the exclusion of thealignment process and reduction of viewing angle dependence. Inaddition, since an alignment film does not need to be provided andrubbing treatment is unnecessary, electrostatic discharge damage causedby the rubbing treatment can be prevented and defects and damage of theliquid crystal display device can be reduced in the manufacturingprocess. Thus, productivity of the liquid crystal display device can beincreased.

The specific resistivity of the liquid crystal material is greater thanor equal to 1×10⁹ Ω·cm, preferably greater than or equal to 1×10¹¹ Ω·cm,more preferably greater than or equal to 1×10¹² Ω·cm. Note that thespecific resistance in this specification is measured at 20° C.

The size of storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. The size of the storage capacitor may be setconsidering the off-state current of a transistor or the like. By usinga transistor including an oxide semiconductor layer, which is disclosedin this specification, it is enough to provide a storage capacitorhaving a capacitance that is ⅓ or less, preferably ⅕ or less of liquidcrystal capacitance of each pixel.

In the transistor including an oxide semiconductor layer, which isdisclosed in this specification, the current in an off state (off-statecurrent) can be made small. Accordingly, an electric signal such asimage data can be held for a longer period and a writing interval can beset longer. Accordingly, the frequency of refresh operation can bereduced, which leads to an effect of suppressing power consumption.

The transistor including an oxide semiconductor layer, which isdisclosed in this specification, can have relatively high field-effectmobility; thus, the driver circuit can operate at high speed. Forexample, when such a transistor is used for a liquid crystal displaydevice, a switching transistor in a pixel portion and a drivertransistor in a driver circuit portion can be formed over one substrate.In addition, by using such a transistor in a pixel portion, ahigh-quality image can be provided.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. Some examples are given as the vertical alignment mode.For example, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, an Advanced Super View (ASV) mode, andthe like can be used. Furthermore, this embodiment can be applied to aVA liquid crystal display device. The VA liquid crystal display devicehas a kind of form in which alignment of liquid crystal molecules of aliquid crystal display panel is controlled. In the VA liquid crystaldisplay device, liquid crystal molecules are aligned in a verticaldirection with respect to a panel surface when no voltage is applied.Moreover, it is possible to use a method called domain multiplication ormulti-domain design, in which a pixel is divided into some regions(subpixels) and molecules are aligned in different directions in theirrespective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method in the pixel portion, a progressive method, aninterlace method or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite); R, G, B, and one or more of yellow, cyan, magenta, and the like;or the like can be used. Further, the sizes of display regions may bedifferent between respective dots of color elements. Note that oneembodiment of the disclosed invention is not limited to the applicationto a display device for color display; the disclosed invention can alsobe applied to a display device for monochrome display.

FIGS. 11A to 11C illustrate examples in which a common connectionportion (pad portion) for electrically connection with the secondelectrode layer 4031 provided on the substrate 4006 in the displaydevice in FIG. 5 is formed over the substrate 4001.

The common connection portion is provided in a position that overlapswith a sealant for bonding the substrate 4001 and the substrate 4006,and is electrically connected to the second electrode layer 4031 viaconductive particles contained in the sealant. Alternatively, the commonconnection portion is provided in a position that does not overlap withthe sealant (except for the pixel portion) and a paste containingconductive particles is provided separately from the sealant so as tooverlap with the common connection portion, whereby the commonconnection portion is electrically connected to the second electrodelayer 4031.

FIG. 11A is a cross-sectional view of the common connection portion andcorresponds to a cross section along G1-G2 in the top view in FIG. 11B.

A common potential line 491 is provided over a gate insulating layer4020, and is formed using a material and a process similar to those ofthe source and drain electrodes of the transistors 4010 and 4011illustrated in FIG. 5.

Further, the common potential line 491 is covered with the insulatinglayer 4032, and the insulating layer 4032 includes a plurality ofopening portions overlapping with the common potential line 491. Theopening portions are formed in the same process as a contact hole forconnecting one of the source and drain electrodes of the transistor 4010and the first electrode layer 4034.

A common electrode 492 is provided over the insulating layer 4032 andformed using a material and a process similar to those of the connectionterminal electrode 4015 and the first electrode layer 4034 of the pixelportion.

In this manner, the common connection portion can be formed by utilizingthe process for manufacturing the switching element of the pixel portion4002.

Note that the common electrode 492 is an electrode in contact with theconductive particles contained in the sealant, and is electricallyconnected to the second electrode layer 4031 of the substrate 4006.

As illustrated in FIG. 11C, the common potential line 491 may be formedusing a material and a process similar to those of the gate electrodesof the transistors 4010 and 4011.

In the common connection portion illustrated in FIG. 11C, the commonpotential line 491 is provided below the gate insulating layer 4020 andthe insulating layer 4032, and the gate insulating layer 4020 and theinsulating layer 4032 have a plurality of opening portions overlappingwith the common potential line 491. The opening portions are formed insuch a manner that the insulating layer 4032 is etched in the sameprocess as the contact hole for connecting one of the source and drainelectrode layers of the transistor 4010 and the first electrode layer4034, and then the gate insulating layer 4020 is selectively etched.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element. In this embodiment, an organic EL element isused as a light-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

In order to extract light emitted from the light-emitting element, atleast one of the pair of electrodes has a light-transmitting property.The light-emitting element can have a top emission structure in whichlight emission is extracted through the surface opposite to thesubstrate; a bottom emission structure in which light emission isextracted through the surface on the substrate side; or a dual emissionstructure in which light emission is extracted through the surfaceopposite to the substrate and the surface on the substrate side, and alight-emitting element having any of these emission structures can beused.

FIGS. 6A and 6B and FIG. 13 illustrate an example of a light-emittingdevice including the light-emitting element as a display element.

FIG. 6A is a plan view of the light-emitting device, and FIG. 6B is across-sectional view taken along dashed-dotted lines S1-T1, S2-T2, andS3-T3 in FIG. 6A. FIG. 13 is a cross-sectional view taken along adashed-dotted S4-T4 in FIG. 6A. Note that, an electroluminescent layer542 and a second electrode layer 543 are not illustrated in the planview in FIG. 6A.

The light-emitting device illustrated in FIGS. 6A and 6B includes, overa substrate 500, a transistor 510, a capacitor 520, and a wiring layerintersection 530. The transistor 510 is electrically connected to alight-emitting element 540. Note that FIGS. 6A and 6B illustrate abottom-emission light-emitting device in which light from thelight-emitting element 540 is extracted through the substrate 500.

The transistor described in Embodiment 1 or 2 can be applied to thetransistor 510. In this embodiment, an example in which a transistorhaving a structure similar to that of the transistor 330 described inEmbodiment 2 is used is described. The transistor 510 is a bottom-gatetransistor.

The transistor 510 includes gate electrode layers 511 a and 511 b; agate insulating layer 502 including gate insulating layers 502 a, 502 b,and 502 c; an oxide semiconductor layer 512 including oxidesemiconductor layers 512 b and 512 a; and conductive layers 513 a and513 b serving as a source electrode layer and a drain electrode layer.Further, an insulating layer 525 is formed over the transistor 510.

The capacitor 520 includes conductive layers 521 a and 521 b, the gateinsulating layer 502, an oxide semiconductor stack 522 including oxidesemiconductor layers 522 b and 522 a, and a conductive layer 523. Thegate insulating layer 502 and the oxide semiconductor stack 522 aresandwiched between the conductive layer 523 and the conductive layers521 a and 521 b, whereby the capacitor is formed.

The wiring layer intersection 530 is an intersection of a conductivelayer 533 and the gate electrode layers 511 a and 511 b. The conductivelayer 533 and the gate electrode layers 511 a and 511 b intersect witheach other with the gate insulating layer 502 provided therebetween.

In this embodiment, a 30-nm-thick titanium film is used as each of thegate electrode layer 511 a and the conductive layer 521 a, and a200-nm-thick copper thin film is used as each of the gate electrodelayer 511 b and the conductive layer 521 b. Thus, the gate electrodelayer has a stacked-layer structure of the titanium film and the copperthin film.

The transistor 510 includes the oxide semiconductor layer 512 bfunctioning as a channel, and the oxide semiconductor layer 512 afunctioning as a buffer layer for preventing diffusion of a constituentelement of the conductive layers 513 a and 513 b into the channel.Accordingly, the transistor 510 is a highly reliable transistor in whichinfluence of an interface state which can be formed on the back channelside is reduced.

Further, in the transistor 510, a nitrogen-containing silicon film whichfunctions as a barrier film against copper and whose ammonia content isreduced is included as the gate insulating layer 502 c, anitrogen-containing silicon film having a large thickness (e.g., 300 nm)in which the number of defects is reduced is included as the gateinsulating layer 502 a, and a nitrogen-containing silicon film in whichthe hydrogen concentration is reduced is included as the gate insulatinglayer 502 b. With such a structure, the transistor 510 can havefavorable electrical characteristics and electrostatic breakdown of thetransistor 510 can be prevented. Accordingly, a highly reliablesemiconductor device can be provided in a high yield.

An interlayer insulating layer 504 is formed over the transistor 510,the capacitor 520, and the wiring layer intersection 530. Over theinterlayer insulating layer 504, a color filter layer 505 is provided ina region overlapping with the light-emitting element 540. An insulatinglayer 506 functioning as a planarization insulating layer is providedover the interlayer insulating layer 504 and the color filter layer 505.

The light-emitting element 540 having a stacked-layer structure in whicha first electrode layer 541, the electroluminescent layer 542, and thesecond electrode layer 543 are stacked in this order is provided overthe insulating layer 506. The first electrode layer 541 and theconductive layer 513 a are in contact with each other in an openingformed in the insulating layer 506 and the interlayer insulating layer504, which reaches the conductive layer 513 a; thus the light-emittingelement 540 and the transistor 510 are electrically connected to eachother. Note that a partition 507 is provided so as to cover part of thefirst electrode layer 541 and the opening.

Further, a 1500-nm-thick photosensitive acrylic film and a 1500-nm-thickphotosensitive polyimide film can be used as the insulating layer 506and the partition 507, respectively.

As the color filter layer 505, for example, a chromaticlight-transmitting resin can be used. As the chromatic colorlight-transmitting resin, a photosensitive organic resin or anon-photosensitive organic resin can be used. The photosensitive organicresin is preferably used, in which case the number of resist masks canbe reduced, which results in the simplification of the process.

Chromatic colors are all colors except achromatic colors such as black,gray, and white. The color filter layer is formed using a material whichtransmits only light of the chromatic colors. As chromatic color, red,green, blue, or the like can be used. Alternatively, cyan, magenta,yellow, or the like may also be used. “Transmitting only light of achromatic color” means that light passing through the color filter layerhas a peak at a wavelength of the light of the chromatic color. Thethickness of the color filter layer may be controlled as appropriate inconsideration of the relationship between the concentration of thecoloring material to be included and the transmittance of light. Forexample, the color filter layer 505 may have a thickness greater than orequal to 1500 nm and less than or equal to 2000 nm.

The partition 507 can be formed using an organic insulating material oran inorganic insulating material. It is particularly preferable that thepartition 507 be formed using a photosensitive resin material to have anopening over the first electrode layer 541. A sidewall of the opening ispreferably formed as a tilted surface with continuous curvature.

The electroluminescent layer 542 may be formed using either a singlelayer or a stack of a plurality of layers.

A protective film may be formed over the second electrode layer 543 andthe partition 507 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element540. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

Further, the light-emitting element 540 may be covered with a layercontaining an organic compound deposited by an evaporation method sothat oxygen, hydrogen, moisture, carbon dioxide, or the like do notenter the light-emitting element 540.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also called anelectrophoretic display device (electrophoretic display) and hasadvantages in that it has the same level of readability as regularpaper, it has less power consumption than other display devices, and itcan be set to have a thin and light form.

An electrophoretic display device can have various modes. Anelectrophoretic display device includes a plurality of microcapsulesdispersed in a solvent, and each microcapsule contains first particleswhich are positively charged and second particles which are negativelycharged. By applying an electric field to the microcapsules, theparticles in the microcapsules move in opposite directions to each otherand only the color of the particles gathering on one side is displayed.Note that the first particles and the second particles each containpigment and do not move without an electric field. Moreover, the firstparticles and the second particles have different colors (which may becolorless).

A dispersion of the above microcapsules in a solvent is referred to aselectronic ink. Furthermore, by the use of a color filter or particlesthat have a pigment, color display is also possible.

The insulating layer 506 functioning as a planarization insulating layercan be formed using an organic material having heat resistance, such asan acrylic resin, polyimide, a benzocyclobutene-based resin, polyamide,or an epoxy resin. Other than such organic materials, it is alsopossible to use a low-dielectric constant material (low-k material) suchas a siloxane-based resin, phosphosilicate glass (PSG), orborophosphosilicate glass (BPSG). Note that the insulating layer 506 maybe formed by stacking a plurality of insulating layers formed using anyof these materials.

There is no particular limitation on the method of forming theinsulating layer 506; the following method can be used depending on thematerial: a sputtering method, spin coating, dipping, spray coating, adroplet discharge method (an inkjet method), screen printing, offsetprinting, or the like.

Materials similar to those of the first electrode layer 4034 and thesecond electrode layer 4031 illustrated in FIG. 5 can be used for thefirst electrode layer 541 and the second electrode layer 543.

In this embodiment, since the light-emitting device illustrated in FIGS.6A and 6B has a bottom-emission structure, the first electrode layer 541has a light-transmitting property and the second electrode layer 543 hasa light-reflecting property. Accordingly, in the case of using a metalfilm as the first electrode layer 541, the film is preferably thinenough to keep the light-transmitting property; on the other hand, inthe case of using a light-transmitting conductive film as the secondelectrode layer 543, a conductive layer having a light-reflectingproperty is preferably stacked thereon.

A protective circuit for protecting the driver circuit may be provided.The protection circuit is preferably formed using a nonlinear element.

By using the transistor described in Embodiment 1 or 2 as describedabove, the semiconductor device can have a variety of functions.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the other structures, methods,and the like described in the other embodiments.

Embodiment 4

A semiconductor device having an image sensor function of reading dataon an object can be manufactured using the transistors described inEmbodiment 1 or 2.

An example of a semiconductor device having an image sensor function isillustrated in FIG. 7A. FIG. 7A illustrates an equivalent circuit of aphoto sensor, and FIG. 7B is a cross-sectional view illustrating part ofthe photo sensor.

In a photodiode 602, one electrode is electrically connected to aphotodiode reset signal line 658, and the other electrode iselectrically connected to a gate of a transistor 640. One of a sourceand a drain of the transistor 640 is electrically connected to a photosensor reference signal line 672, and the other of the source and thedrain thereof is electrically connected to one of a source and a drainof a transistor 656. A gate of the transistor 656 is electricallyconnected to a gate signal line 659, and the other of the source and thedrain thereof is electrically connected to a photo sensor output signalline 671.

Note that in circuit diagrams in this specification, a transistor usingan oxide semiconductor layer is denoted by a symbol “OS” so that it canbe identified as a transistor including an oxide semiconductor layer. InFIG. 7A, the transistor 640 and the transistor 656 are each a transistorusing an oxide semiconductor layer, to which the transistor described inEmbodiment 1 or 2 can be applied. This embodiment shows an example ofusing a transistor having a structure similar to that of the transistor320 described in Embodiment 1. The transistor 640 is a bottom-gatetransistor.

FIG. 7B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photosensor. The transistor 640 and the photodiode602 functioning as a sensor are provided over a substrate 601 (anelement substrate) having an insulating surface. A substrate 613 isprovided over the photodiode 602 and the transistor 640 with an adhesivelayer 608 interposed therebetween.

An insulating layer 632, an interlayer insulating layer 633, and aninterlayer insulating layer 634 are provided over the transistor 640.The photodiode 602 includes an electrode layer 641 b formed over theinterlayer insulating layer 633, semiconductor films (a firstsemiconductor film 606 a, a second semiconductor film 606 b, and a thirdsemiconductor film 606 c stacked over the electrode layer 641 b in thisorder), an electrode layer 642 which is provided over the interlayerinsulating layer 634 and electrically connected to the electrode layer641 b through the first to third semiconductor films, and an electrodelayer 641 a which is provided in the same layer as the electrode layer641 b and electrically connected to the electrode layer 642.

The electrode layer 641 b is electrically connected to a conductivelayer 643 formed over the interlayer insulating layer 634, and theelectrode layer 642 is electrically connected to a conductive layer 645through the electrode layer 641 a. The conductive layer 645 iselectrically connected to a gate electrode layer of the transistor 640,and the photodiode 602 is electrically connected to the transistor 640.

Here, a pin photodiode in which a semiconductor film having p-typeconductivity as the first semiconductor film 606 a, a high-resistancesemiconductor film (i-type semiconductor film) as the secondsemiconductor film 606 b, and a semiconductor film having n-typeconductivity as the third semiconductor film 606 c are stacked isillustrated as an example.

The first semiconductor film 606 a is a p-type semiconductor film andcan be formed using an amorphous silicon film containing an impurityelement imparting p-type conductivity type. The first semiconductor film606 a is formed by a plasma CVD method using a semiconductor source gascontaining an impurity element belonging to Group 13 (e.g., boron (B)).As the semiconductor material gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film using a diffusion methodor an ion injecting method. Heating or the like may be conducted afterintroducing the impurity element by an ion injecting method or the likein order to diffuse the impurity element. In this case, as a method offorming the amorphous silicon film, an LPCVD method, a chemical vapordeposition method, a sputtering method, or the like may be used. Thefirst semiconductor film 606 a is preferably formed to have a thicknessgreater than or equal to 10 nm and less than or equal to 50 nm.

The second semiconductor film 606 b is an i-type semiconductor film(intrinsic semiconductor film) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor film 606 b, anamorphous silicon film is formed by a plasma CVD method using asemiconductor source gas. As the semiconductor material gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. The second semiconductor film 606 b may beformed by an LPCVD method, a vapor deposition method, a sputteringmethod, or the like. The second semiconductor film 606 b is preferablyformed to have a thickness greater than or equal to 200 nm and less thanor equal to 1000 nm.

The third semiconductor film 606 c is an n-type semiconductor film andis formed using an amorphous silicon film containing an impurity elementimparting n-type conductivity type. The third semiconductor film 606 cis formed by a plasma CVD method using a semiconductor source gascontaining an impurity element belonging to Group 15 (e.g., phosphorus(P)). As the semiconductor material gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with use of a diffusionmethod or an ion injecting method. Heating or the like may be conductedafter introducing the impurity element by an ion injecting method or thelike in order to diffuse the impurity element. In this case, as a methodof forming the amorphous silicon film, an LPCVD method, a chemical vapordeposition method, a sputtering method, or the like may be used. Thethird semiconductor film 606 c is preferably formed to have a thicknessgreater than or equal to 20 nm and less than or equal to 200 nm.

The first semiconductor film 606 a, the second semiconductor film 606 b,and the third semiconductor film 606 c are not necessarily formed usingan amorphous semiconductor, and may be formed using a polycrystallinesemiconductor or a microcrystalline semiconductor (semi-amorphoussemiconductor: SAS).

The mobility of holes generated by the photoelectric effect is lowerthan the mobility of electrons. Therefore, a PIN photodiode has bettercharacteristics when a surface on the p-type semiconductor film side isused as a light-receiving plane. Here, an example in which lightreceived by the photodiode 602 from a surface of the substrate 601, overwhich the pin photodiode is formed, is converted into electric signalsis described. Further, light from the semiconductor film having theconductivity type opposite to that of the semiconductor film on thelight-receiving plane is disturbance light; therefore, the electrodelayer is preferably formed using a light-blocking conductive layer. Notethat the n-type semiconductor film side may alternatively be alight-receiving plane.

The transistor 640 includes a first oxide semiconductor layer whichfunctions as a buffer layer for preventing diffusion of a constituentelement of the source and drain electrode layers into a channel, asecond oxide semiconductor layer which functions as the channel, and athird oxide semiconductor layer which is provided between a gateinsulating layer 631 and the oxide semiconductor layer functioning asthe channel and functions as a buffer layer for preventing degradationof the interface on the gate insulating layer 631 side of the channel.Accordingly, the transistor 640 is a highly reliable transistor in whichinfluence of an interface state which can be formed on the back channelside is reduced and photodegradation (e.g., negative-bias temperaturestress photodegradation) is reduced.

The insulating layer 632, the interlayer insulating layer 633, and theinterlayer insulating layer 634 can be formed using an insulatingmaterial by a sputtering method, a plasma CVD method, spin coating,dipping, spray coating, a droplet discharge method (such as an inkjetmethod), a printing method (such as screen printing or offset printing),or the like depending on the material.

For reduction of surface roughness, an insulating layer functioning as aplanarization insulating layer is preferably used as each of theinterlayer insulating layers 633 and 634. For the interlayer insulatinglayers 633 and 634, for example, an organic insulating material havingheat resistance, such as polyimide, an acrylic resin, abenzocyclobutene-based resin, polyamide, or epoxy, can be used. Otherthan such organic insulating materials, it is possible to use a singlelayer or stacked layers of a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like.

With detection of light that enters the photodiode 602, data on anobject can be read. Note that a light source such as a backlight can beused at the time of reading information on an object.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the other structures, methods,and the like described in the other embodiments.

Embodiment 5

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof electronic appliances include a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, cameras such as a digital camera and a digital video camera, adigital photo frame, a mobile phone, a portable game machine, a portableinformation terminal, an audio reproducing device, a game machine (e.g.,a pachinko machine or a slot machine), a game console, and the like.Specific examples of these electronic appliances are illustrated inFIGS. 8A to 8C.

FIG. 8A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, a powercord 9005 for supplying power is provided for the housing 9001.

The semiconductor device described in any of the above embodiments canbe used in the display portion 9003, so that the electronic appliancecan have high reliability.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her finger or the like, the user can carryout operation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the table 9000 may function as a control device whichcontrols the home appliances by operation on the screen. For example,with use of the semiconductor device having an image sensor described inEmbodiment 3, the display portion 9003 can function as a touch panel.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 8B illustrates a television set 9100. In the television set 9100, adisplay portion 9103 is incorporated in a housing 9101 and an image canbe displayed on the display portion 9103. Note that the housing 9101 issupported by a stand 9105 here.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with an operation key 9109 of the remote controller9110 so that an image displayed on the display portion 9103 can becontrolled. Furthermore, the remote controller 9110 may be provided witha display portion 9107 for displaying data output from the remotecontroller 9110.

The television set 9100 illustrated in FIG. 8B is provided with areceiver, a modem, and the like. With the use of the receiver, thetelevision set 9100 can receive general TV broadcasts. Moreover, whenthe television set 9100 is connected to a communication network with orwithout wires via the modem, one-way (from a sender to a receiver) ortwo-way (between a sender and a receiver or between receivers)information communication can be performed.

The semiconductor device described in any of the above embodiments canbe used in the display portions 9103 and 9107, so that the televisionset and the remote controller can have high reliability.

FIG. 8C illustrates a computer which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

The semiconductor device described in any of the above embodiments canbe used in the display portion 9203, so that the computer can have highreliability.

FIGS. 9A and 9B illustrate a tablet terminal that can be folded. In FIG.9A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a switch 9034 forswitching display modes, a power switch 9035, a switch 9036 forswitching to power-saving mode, a clip 9033, and an operation switch9038.

The semiconductor device described in any of the above embodiments canbe used in the display portion 9631 a and the display portion 9631 b, sothat the tablet terminal can have high reliability.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 9A shows, as an example, that half of the areaof the display portion 9631 a has only a display function and the otherhalf of the area has a touch panel function. However, the structure ofthe display portion 9631 a is not limited to this, and all the area ofthe display portion 9631 a may have a touch panel function. For example,all the area of the display portion 9631 a can display keyboard buttonsand serve as a touch panel while the display portion 9631 b can be usedas a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The switch 9034 for switching display modes allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. With the switch 9036 for switching topower-saving mode, the luminance of display can be optimized inaccordance with the amount of external light at the time when the tabletterminal is in use, which is detected with an optical sensorincorporated in the tablet terminal. The tablet terminal may includeanother detection device such as a sensor for detecting orientation(e.g., a gyroscope or an acceleration sensor) in addition to the opticalsensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 9A, an embodiment of the present inventionis not limited to this example. The display portion 9631 a and thedisplay portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

FIG. 9B illustrates the tablet terminal folded, which includes thehousing 9630, a solar battery 9633, and a charge and discharge controlcircuit 9634. Note that FIG. 9B shows an example in which the charge anddischarge control circuit 9634 includes the battery 9635 and the DCDCconverter 9636.

Since the tablet can be folded in two, the housing 9630 can be closedwhen the tablet is not in use. Thus, the display portions 9631 a and9631 b can be protected, thereby providing a tablet with high enduranceand high reliability for long-term use.

The tablet terminal illustrated in FIGS. 9A and 9B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or both surfaces of the housing 9630 and thebattery 9635 can be charged efficiently. When a lithium ion battery isused as the battery 9635, there is an advantage of downsizing or thelike.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 9B are described with reference to a blockdiagram of FIG. 9C. FIG. 9C illustrates the solar battery 9633, thebattery 9635, the DCDC converter 9636, a converter 9637, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DCDC converter9636, the converter 9637, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 in FIG. 9B.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by theDCDC converter 9636 so that a voltage for charging the battery 9635 isobtained. When the display portion 9631 is operated with the power fromthe solar battery 9633, the switch SW1 is turned on and the voltage ofthe power is raised or lowered by the converter 9637 to a voltage neededfor operating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off anda switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Example

In this example, measurement results of the concentrations of aconstituent element of a source electrode layer or a drain electrodelayer which is contained in stacked oxide semiconductor layers in atransistor are described. Specifically, an electrode layer including acopper film was formed for a source electrode layer and a drainelectrode layer and the copper concentration in each oxide semiconductorlayer was measured. For the measurement of the copper concentration,substrate side depth profile secondary ion mass spectrometry (SSDP-SIMS)was used.

A method for forming a transistor used in the measurement in thisexample is described below. In this example, a transistor having astructure similar to that of the transistor 300 in Embodiment 1 wasformed. The same reference numerals as those of the transistor 300 areused in the following description.

First, a 100-nm-thick tungsten film was formed as the gate electrodelayer 402 over the substrate 400.

Next, a 50-nm-thick silicon nitride film and a 200-nm-thick siliconoxynitride film were formed successively by a plasma CVD method as thegate insulating layer 404 covering the gate electrode layer 402.

The silicon nitride film was deposited in a condition where the pressurein a treatment chamber of a plasma CVD apparatus was set to 60 Pa, apower of 150 W was supplied at a high frequency power source of 27.12MHz, the substrate temperature was set to 350° C., and a mixed gas ofsilane and nitrogen (SiH₄:N₂=50 sccm:5000 sccm) was supplied. Note thatthe plasma CVD apparatus is a parallel plate plasma CVD apparatus havingan electrode area of 6000 cm². The silicon oxynitride film was depositedin the same treatment chamber in a condition where the pressure was setto 40 Pa, the high-frequency power source and the substrate temperaturewere maintained, and a mixed gas of silane and dinitrogen monoxide(SiH₄:N₂O=20 sccm:3000 sccm) was supplied.

Next, the oxide semiconductor layer 408 b with a thickness of 10 nm wasformed over the gate insulating layer 404 by a sputtering method using ametal oxide target having an atomic ratio of In:Ga:Zn=1:1:1. Thedeposition conditions are as follows: the atmosphere contains oxygen at50%, the pressure was 0.6 Pa, the electric power was 5 kW, and thesubstrate temperature was 170° C.

After that, the oxide semiconductor layer 408 a with a thickness of 30nm was formed over the oxide semiconductor layer 408 b by a sputteringmethod using a metal oxide target having an atomic ratio ofIn:Ga:Zn=3:1:2. The deposition conditions are as follows: the atmospherecontains oxygen at 50%, the pressure was 0.6 Pa, the electric power was5 kW, and the substrate temperature was 170° C.

Processing was performed so that the island-shaped oxide semiconductorstack 408 was obtained. Then, the oxide semiconductor stack 408 wassubjected to heat treatment in a nitrogen atmosphere at 450° C. for onehour and then subjected to heat treatment in an atmosphere containingnitrogen and oxygen at 450° C. for one hour.

Next, the source electrode layer 410 a and the drain electrode layer 410b which were in contact with the oxide semiconductor stack 408 wereformed.

In this example, the source electrode layer 410 a and the drainelectrode layer 410 b were formed in such a manner that a 35-nm-thicktitanium film and a 200-nm-thick copper film were stacked over the gateinsulating layer 404 and the oxide semiconductor stack 408 and part ofthe titanium film and part of the copper film were selectively etched.

After that, a 400-nm-thick silicon oxynitride film was deposited by aplasma CVD method as the insulating layer 412 covering the oxidesemiconductor stack 408, the source electrode layer 410 a, and the drainelectrode layer 410 b.

The silicon oxynitride film was deposited in a condition where thepressure of a treatment chamber of a plasma CVD apparatus was controlledto 200 Pa, a power of 1500 W was supplied at a high frequency powersource of 27.12 MHz, the substrate temperature was set to 220° C., and amixed gas of silane and dinitrogen monoxide (SiH₄:N₂O=160 sccm:4000sccm) was supplied.

Heat treatment was performed in an atmosphere containing nitrogen andoxygen at 300° C. for one hour, and then a 1.5-μm-thick acrylic resinfilm was formed as a planarization film. After that, heat treatment wasperformed in a nitrogen atmosphere at 250° C. for one hour.

Through the above process, the transistor of this example was formed.

The copper concentration of the oxide semiconductor stack 408 includedin the obtained transistor was measured by SSDP-SIMS. The measurementresults by SSDP-SIMS are shown in FIG. 12. Note that the measurement wasperformed on a region between the source electrode layer 410 a and thedrain electrode layer 410 b (a region where a channel is formed).

FIG. 12 shows that diffusion of copper which is a constituent element ofthe source electrode layer 410 a and the drain electrode layer 410 binto the oxide semiconductor stack 408 was observed but copper was notdiffused beyond the oxide semiconductor layer 408 a. Thus, it wasconfirmed that the copper does not reach the oxide semiconductor layer408 b, which functions as a current path (channel) of the transistor.

The above results show that, in the oxide semiconductor layer 408, theoxide semiconductor layer 408 a can function as a buffer layer forpreventing diffusion of the constituent element of the source electrodelayer 410 a and the drain electrode layer 410 b by being provided on theback channel side of the oxide semiconductor layer 408 b serving as acurrent path. Accordingly, it is suggested that variations in theelectrical characteristics of the transistor including the oxidesemiconductor stack 408 can be prevented.

REFERENCE NUMERALS

300: transistor; 310: transistor; 320: transistor; 330: transistor; 400:substrate; 402: gate electrode layer; 403: gate insulating layer; 403 a:gate insulating layer; 403 b: gate insulating layer; 403 c: gateinsulating layer; 404: gate insulating layer; 406: gate insulatinglayer; 408: oxide semiconductor stack; 408 a: oxide semiconductor layer;408 b: oxide semiconductor layer; 408 c: oxide semiconductor layer; 410a: source electrode layer; 410 b: drain electrode layer; 412: insulatinglayer; 491: common potential line; 492: common electrode; 500:substrate; 502: gate insulating layer; 502 a: gate insulating layer; 502b: gate insulating layer; 502 c: gate insulating layer; 504: interlayerinsulating layer; 505: color filter layer; 506: insulating layer; 507:partition; 510: transistor; 511 a: gate electrode layer; 511 b: gateelectrode layer; 512: oxide semiconductor stack; 512 a: oxidesemiconductor layer; 512 b: oxide semiconductor layer; 513 a: conductivelayer; 513 b: conductive layer; 520: capacitor; 521 a: conductive layer;521 b: conductive layer; 522: oxide semiconductor stack; 522 a: oxidesemiconductor layer; 522 b: oxide semiconductor layer; 523: conductivelayer; 525: insulating layer; 530: wiring layer intersection; 533:conductive layer; 540: light-emitting element; 541: electrode layer;542: electroluminescent layer; 543: electrode layer; 601: substrate;602: photodiode; 606 a: semiconductor film; 606 b: semiconductor film;606 c: semiconductor film; 608: adhesive layer; 613: substrate; 631:gate insulating layer; 632: insulating layer; 633: interlayer insulatinglayer; 634: interlayer insulating layer; 640: transistor; 641 a:electrode layer; 641 b: electrode layer; 642: electrode layer; 643:conductive layer; 645: conductive layer; 656: transistor; 658:photodiode reset signal line; 659: gate signal line; 671: photo sensoroutput signal line; 672: photo sensor reference signal line; 4001:substrate; 4002: pixel portion; 4003: signal line driver circuit; 4004:scan line driver circuit; 4005: sealant; 4006: substrate; 4008: liquidcrystal layer; 4010: transistor; 4011: transistor; 4013: liquid crystalelement; 4015: connection terminal electrode; 4016: terminal electrode;4018: FPC; 4019: anisotropic conductive layer; 4020: gate insulatinglayer; 4031: electrode layer; 4032: insulating layer; 4033: insulatinglayer; 4034: electrode layer; 4035: spacer; 4038: insulating layer;9000: table; 9001: housing; 9002: leg portion; 9003: display portion;9004: displayed button; 9005: power cord; 9033: clip; 9034: switch;9035: switch; 9036: switch; 9038: operation switch; 9100: televisionset; 9101: housing; 9103: display portion; 9105: stand; 9107: displayportion; 9109: operation key; 9110: remote controller; 9201: main body;9202: housing; 9203: display portion; 9204: keyboard; 9205: externalconnection port; 9206: pointing device; 9630: housing; 9631: displayportion; 9631 a: display portion; 9631 b: display portion; 9632 a:region; 9632 b: region; 9633: solar battery; 9634: charge and dischargecontrol circuit; 9635: battery; 9636: DCDC converter; 9637: converter;9638: operation key; 9639: button.

This application is based on Japanese Patent Application serial no.2012-125432 filed with Japan Patent Office on May 31, 2012, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a gate electrode layer; a gateinsulating layer over the gate electrode layer; a first oxidesemiconductor layer over the gate insulating layer, the first oxidesemiconductor layer comprising indium and gallium; a second oxidesemiconductor layer over the first oxide semiconductor layer, the secondoxide semiconductor layer comprising indium and gallium; a sourceelectrode layer over the second oxide semiconductor layer; and a drainelectrode layer over the second oxide semiconductor layer, wherein acontent of the indium is higher than a content of the gallium in thefirst oxide semiconductor layer, wherein a content of the indium islower than or equal to a content of the gallium in the second oxidesemiconductor layer, and wherein the second oxide semiconductor layercomprises at least one element of the source electrode layer and thedrain electrode layer.
 2. The semiconductor device according to claim 1,wherein the source electrode layer and the drain electrode layer containcopper.
 3. The semiconductor device according to claim 1, wherein thegate insulating layer includes a silicon nitride film.
 4. Thesemiconductor device according to claim 1, wherein each of the firstoxide semiconductor layer and the second oxide semiconductor layercomprises zinc.
 5. The semiconductor device according to claim 1,wherein the semiconductor device is a display device.
 6. A semiconductordevice comprising: a gate electrode layer; a gate insulating layer overthe gate electrode layer; a first oxide semiconductor layer over thegate insulating layer, the first oxide semiconductor layer comprisingindium and gallium; a second oxide semiconductor layer over the firstoxide semiconductor layer, the second oxide semiconductor layercomprising indium and gallium; a third oxide semiconductor layer overthe second oxide semiconductor layer, the third oxide semiconductorlayer comprising indium and gallium; a source electrode layer inelectrical contact with the second oxide semiconductor layer; and adrain electrode layer in electrical contact with the second oxidesemiconductor layer, wherein a content of the indium is lower than orequal to a content of the gallium in the first oxide semiconductorlayer, wherein a content of the indium is higher than a content of thegallium in the second oxide semiconductor layer, wherein a content ofthe indium is lower than or equal to a content of the gallium in thethird oxide semiconductor layer, and wherein the third oxidesemiconductor layer comprises at least one element of the sourceelectrode layer and the drain electrode layer.
 7. The semiconductordevice according to claim 6, wherein the source electrode layer is overthe third oxide semiconductor layer, and wherein the drain electrodelayer is over the third oxide semiconductor layer.
 8. The semiconductordevice according to claim 6, wherein the source electrode layer and thedrain electrode layer contain copper.
 9. The semiconductor deviceaccording to claim 6, wherein the gate insulating layer includes asilicon nitride film.
 10. The semiconductor device according to claim 6,wherein each of the first oxide semiconductor layer, the second oxidesemiconductor layer and the third oxide semiconductor layer compriseszinc.
 11. The semiconductor device according to claim 6, wherein thesemiconductor device is a display device.
 12. A semiconductor devicecomprising: a gate electrode layer; a gate insulating layer over thegate electrode layer; a first oxide semiconductor layer over the gateinsulating layer, the first oxide semiconductor layer comprising indiumand gallium; a second oxide semiconductor layer over the first oxidesemiconductor layer, the second oxide semiconductor layer comprisingindium and gallium; a third oxide semiconductor layer over the secondoxide semiconductor layer, the third oxide semiconductor layercomprising indium and gallium; a source electrode layer in electricalcontact with the second oxide semiconductor layer; and a drain electrodelayer in electrical contact with the second oxide semiconductor layer,wherein a content of the indium is lower than or equal to a content ofthe gallium in the first oxide semiconductor layer, wherein a content ofthe indium is higher than a content of the gallium in the second oxidesemiconductor layer, wherein a content of the indium is lower than orequal to a content of the gallium in the third oxide semiconductorlayer, wherein the third oxide semiconductor layer comprises at leastone element of the source electrode layer and the drain electrode layer,and wherein the first oxide semiconductor layer comprises at least oneelement of the gate insulating layer.
 13. The semiconductor deviceaccording to claim 12, wherein the source electrode layer is over thethird oxide semiconductor layer, and wherein the drain electrode layeris over the third oxide semiconductor layer.
 14. The semiconductordevice according to claim 12, wherein the source electrode layer and thedrain electrode layer contain copper.
 15. The semiconductor deviceaccording to claim 12, wherein the gate insulating layer includes asilicon nitride film.
 16. The semiconductor device according to claim12, wherein each of the first oxide semiconductor layer, the secondoxide semiconductor layer and the third oxide semiconductor layercomprises zinc.
 17. The semiconductor device according to claim 12,wherein the semiconductor device is a display device.